Introduction

Before choosing things to mod, you have to be somewhat familiar with the Official Ranking Criteria and Standard Ranking Criteria which lists the rules and guidelines about mapping, their purpose and their explanations and application. This should be your technical modding criteria as well, because based on this you will most likely point out things. More on this later.

Before modding a map of your choice, you should really check the map's thread for other mod posts. There is a lot of very useful information located in those threads which usually may save your time, effort and nerves. If you are modding a map of a person you don't know, you may also find out how the person acts towards mods and based on this you can shape your suggestions so they don't get discarded without consideration.
This is not a necessity, but it does help a little on occasion.

When modding you need to be able to present your mod to a person so they understand it. You don't have to make the best looking format of your post ever, but make your mod somewhat easy to understand and navigate in (I should learn from this as well haha). Just think of some basic structure which groups relevant things up, so they are easy to catch on.

There is one very critical thing which I noticed very few people care about and that is it is always best to explain your suggestions or reasons behind changes in your mod post. I just can't stress this enough!
This is very important because if you just say things without reason (such as "Move this circle to x:250 y:250", or "add finish"), it will not be as helpful, or helpful at all. Behind every change there just has to be a reason.
You don't have to write essays in your mod post, but you really need to explain why is your change better than the original. This isn't hard to do, at least not as hard as I may make it sound, because 99% of the time that reason is backed up by the map's music itself.
Your ideas of a change should not be random either, because it is not you who is mapping, so suggesting things for the sake of a change rather than your vision of improvement is not what you should be doing. You can easily explain your vision of improvement on the spot or as a short comment anywhere in your mod post.

tldr of the above - When making a suggestion, explain your reasoning and if possible provide an alternative to reach a compromise.

This should conclude the introduction! Lets start with some basic things which are extremely easy to check!

Step 1 - Check the folder and the files in the folder

This is by far the easiest thing to do. In the folder, you always check for the standard folder structure of a beatmap. It should include an mp3 file, .osu files for every difficulty, .osb if it has a storyboard, a .jpg or .png background picture,some hitsounds (not always), and probably even a video (.avi or .flv).
What you are looking for here is that you should see only files that are relevant to the map. Sometimes there happen to be extra or unused files in the folder, or maybe even files missing. In this step you may not notice this at first, but those cases are quite rare and you can always go back to this step if you notice some things don't work as they should in other steps.

In this step you can also check for things which are easy to catch and are not directly related just to the listing of the files. For example, while viewing the contents of the folder, you can also check the bitrate of the mp3, the dimensions of a background, or if the hitsounds are proper.
For maps with a storyboard and/or a skin, you can also check the dimensions of the pictures used and point out if they are too large.

You can also check every .osu file manually for metadata. This brings us to the step 2.

Step 2 - Metadata and General Information

This step is directly linked to the step 1, because you can go to this step while still on step 1.

Basically, you open .osu files one by one in a text editor, such as notepad, and you compare things there.

In this step, what you are looking for is for the differences in the [General] and [Metadata] fields.
Things that just have to be the same in all difficulties are the following:

1. [General] section:
   1. AudioFilename, unless there are multiple audio files relevant to your map (possible compilations or similar), however in more than 99% cases (I believe), only one mp3 is present and used.
   2. PreviewTime, unless there is a different mp3 used.

1. [Metadata] section:
   1. Everything except for BeatmapID. This one is decided by the osu itself generated when a map is getting submitted and shouldn't be touched. If it's not a submitted map, the value will most likely be 0 and the BeatmapSetID will be -1.

While the above ensures that everything is the same, there is another issue here you need to watch out for and that is the correct song title, artist and source.
To check these, you will have to utilize your own resources (if you know sites that are relevant to the song or where the song is coming from), or just use google. While people believe googling is tedious to do, it's also a very easy way to reach correct information with some effort.

Notice that both step1 and step2 don't even require osu!'s Edit Mode, or even having the client running, so this is one of the very easy things to check when you want to check for things and are unable to run the client for whatever the reason. You can check for the title and the source from the in game song selection menu as well, it's probably faster too, but since we're doing this to make it as accessible as possible, notepad is a safer solution because you get more information rather than just the artist, title and source.

Step 3 - Timing

From this step onward, we will use the osu! Edit Mode.

This step is mostly self explanatory and I will not provide the guide on how to properly time things. For that, check out a great guide made by Charles445 and view it here.

In this step, you need to make sure the timing is the same throughout all the maps in the mapset. Since most of the songs have just a single BPM, this is very easy to check from the Timing tab, because you can open any map in the mapset without leaving that tab. Just make sure the BPM and offset are the same on every difficulty. For those with more than one Offset or BPM, you have to use the Timing menu in the drop down options at the top instead and compare. You should check the Timing menu as well because it will show what kind of timing signature the timing section has (3/4, 4/4, 5/4, etc.). This might seem redundant but there have been cases where people had correct BPM and offset, but wrong timing signature on occasion.

Next, you need to make sure the timing is more or less correct. This means the timing has to have the correct BPM, Offset and Time Signature. Experience with timing plays a major role here and if you're not experienced chances are you may not give the correct timing, but you can always check the current timing and point out if it feels off, bad and/or wrong. Timing signature should be watched out for especially since you can't see it on the Timing Tab. This also includes metronome resets (which means there is more than one Offset of the same BPM, typically located on a tick of the timeline, usually white or red).

There are times where people choose to double or halve BPM for whatever purpose, this requires a lot of musical knowledge and experience to detect properly and if you are not absolutely sure what you are doing, just leave it as a comment instead of an issue you are pointing out.

In this step, I will not cover the importance of the Inherited Timing Sections (commonly known as "Green Lines") because that's a process which extends a style or the mapper's intentions, more on this later.

Step 3.1 - Timing - Note Snapping

This is both easy and hard to check, I will only list the easy way, I will explain the hard way later. Both are necessary.
Basically, you need to check for unsnapped notes. They occur because the timing was most probably modified at some point, but the notes didn't get resnapped!
To do this, you should always check AI Mod and see if it is reporting any unsnapped notes. Be sure to note this, if there are many of those, point out that a resnap should be done probably, else just list the unsnapped ones.

I mentioned the Hard way, and that is the wrongly snapped notes. I will not cover these in this step, more on this later.

Step 4 - On Maps, or more commonly known as Difficulties or Diffs

For simplicity purposes, the term map will be used for an individual difficulty/diff and the term mapset will be used for the whole thing.

This step deals with each map separately and is affected by the person's playing ability, ability to notice things and the personal "approval" of the map.

Best way to mod maps is to testplay them. While this may not uncover all the technical errors present in a map, it will give you a general idea of what the map looks or feels like. Looking at the map is not the same as playing the map. A pretty looking map may be really terrible to play and vice versa.
Note that you don't have to be a skilled player to notice things that are considered wrong or out of place, but one's skill helps immensely. This applies to all game modes!

A lot of people don't know what they should be looking for here, mostly because they don't have their own mapping style, or the mapper's mapping style doesn't have a standard which meets the Ranking Criteria. In this step, you are free to point out everything you find that could use an improvement. This includes slider shapes, the looks of patterns, hitsounds, etc.

I will split this step into more steps which will cover a certain area which deals with those maps.

What is easist to look out for is the new combo placement, spacing and hitsounds.

Step 4.1 - Comboing in Maps

Generally, every combo should be a part of a section as a whole, usually one or two measures (stanzas) long. This may not always be the case (there are times when you can have a single note in a combo which fits, it all depends on what the objects follow in the music), however comboing should not be random.
A lot of mapping styles tend to use their own new combo placements and the purpose of a new combo may have shifted it's meaning, but it's still easy to notice if the comboing makes no sense. Starting a new musical section with, lets say an object with a number 6, doesn't really make much sense.

One other thing which people tend to ignore or not deem quite important is that combo is not used to group up objects into a pattern. It's actually exactly the opposite, the objects inside a combo represent a musical line and the pattern should be shaped based on this, instead of just making a combo pattern.

This may seem like something really advanced and is generally not always followed (or even deemed that wrong), but I assure you, if your objects follow the music, comboing follows the music, and patterns follow your comboing, you just can't go wrong!

What you should also watch out for is combo consistency. If you have 2 very similar, or maybe even the same parts of the music, the comboing should generally be the same, unless the map is mapped differently (for variety purposes, however you can still include variety with different mapping without touching the comboing). It would make little sense to have the first part have a new combo on every measure (for example), and the 2nd part which is the same or similar has a new combo every 2 measures (or, worse, randomly placed anywhere).

When modding comboing, be sure not to spam "new combo, remove new combo" in your mod post, it's tedious to read and uses up precious space. The best way would probably be to generally explain and list the spots which have wrong comboing, or just using spoiler boxes with a list of new combo/remove new combo changes.

Step 4.2 - Spacing

About spacing, a lot of mapping styles don't have consistent spacing, which you should take into account. But is that really fitting?
Random spacing changes may seem cool and innovative, however it is always best to have a somewhat consistent spacing (which doesn't mean the same spacing all the time).

Generally, spacing should probably as consistent as possible within a combo when lining up notes. For example, in the same combo you can have two groups of 3 1/2 snapped notes which are 1/1 apart.

In the both groups you can use the same spacing multiplier (lets say 1.0x), but between each group you could use a different spacing, bigger or smaller. This ensures the map stays fun or challenging, however it should always be kept in moderation that you don't use extreme values.

Based on your testplay of the map, you can easily point out if the spacing feels off, wrong or bad. There are times where spacing can feel extreme, but it could also fit the music, so you have to analyze the song at that part to see if such a spacing fits (this actually means looking for jumps and antijumps).

While that random spacing might still make sense, the moment it trips you or it makes you think it doesn't make sense, don't be afraid to point it out. Just be sure to provide the reason why you don't think it works, and if possible to provide an alternative. This way a compromise can easily be reached and the map may get improved faster.

Nazi mods with 1 grid adjustments are discouraged and should be pointed out only if things are misaligned by mistake and are visible when playing. Feel free to point out things that are considered nazi mods, some mappers actually don't mind those (like myself), however keep in mind that your suggestions might get declined on the spot because some people hate those kind of mods.

One more final thing about spacing, which actually does include comboing too: Generally, a new combo doesn't mean you are free to make a big spacing change of a choice you always want.
A lot of people make a jump or an anti jump and they just justify it with a new combo, this isn't what new combo is for. However, there are many occasions where such a thing actually fits, but that is because the music suggests a new combo there, which also probably suggests a spacing change.
For this you need to analyze the song and the map and you might spend some time on it if you are not experienced.

You don't have to follow this step to the letter to be able to mod spacing, but it's good to know in my opinion.

Step 4.3 - Hitsounds

The usage of hitsounds is extremely wide in variety, however there is always one common thing which can always be checked for, and that is consistency and implementation. If a map sounds bad be sure to point it out. It can have either too many or too few hitsounds, or the hitsounds can be misplaced.

Some music utilizes odd rhythms and hitsound based on that may seem uncommon to a person who doesn't know much about music, however you can always check if a hitsound matches a distinct instrument playing in the music at the spot the hitsound is located on.

Back to the consistency. Consistency is very important, and during the hitsounding process a lot of times what happens is that people just miss and object when applying hitsounds (unless you hitsound every object you map, effectively skipping the "hitsounding process" which is usually done after you mapped your map on the playfield).
To check this, you can either be used to the hitsounds and just notice them on the fly, but if you don't, be sure to check similar parts (most commonly choruses) on how they use hitsounds. You don't have to compare each object, but there is usually a pattern of hitsounding, most commonly utilizing something like every 2nd and 4th white tick to have a clap based sound on. It's easy to spot if such an object is missing a hitsound.
Same goes with finishes which are most commonly used to express most heavy moments in the music, usually being starting or ending notes of a musical section.

Step 5 - On Maps, Part 2 - In depth checking of the Maps

This step is more of a sub-step to step 4, but I have decided to make it have it's own section here because I want to separate very easy and common things from more complex things which will follow.

In this step we will check for more advanced things such as slider speeds, tick rate, hitsound set control, volume control, green timing sections, certain mapping aspects, and similar - in general.

Step 5.1 - Difficulty Settings

Difficulty settings can be subjective, but there are generally acceptable values which should be configured based on the map, rather than the assuming difficulty. The result of difficulty settings might change the star rating in an unwanted way but that does not matter.

Difficulty Settings are as follows:

1. HP Drain. Will refer to it as HP from here onward. This value determines how harsh the health drain is during playing, and how hard is to regain that health back.
2. Circle Size. Will refer to it as CS from here onward. This value determines how small or big the hitcircles or slider head/arrow/tail circles are. Generally, value 4 is most commonly used for most of the maps, but you may find some maps using lower or higher.
3. Approach Rate. Will refer to it as AR from here onward. This value affects how fast the approaching circles are coming towards an object. Generally it's best to have this value set to a certain value where the approach circle speed is similar or identical to the slider speed. More on slider speed later.
4. Overall Difficulty. Will refer to it as OD from here onward. This value affects how difficult the accuracy requirements are to get a 300/100/50 and how hard are spinners to finish. Note that higher OD might indirectly affect how HP works during spinning. In this case it is always better to lower the drain then, instead of OD.

A somewhat general difficulty settings values used for each difficulty type are:

1. Easy: HP2~3, CS2~4, AR2~4, OD2~3.
2. Normal: HP3~5, CS3~4, AR4~5, OD4~5.
3. Hard: HP5~6, CS4~5, AR6~8, OD6~7.
4. Insane: HP6~8, CS4~5, AR7~9, OD7~9.

Keep in mind that the above values are not set in stone, and depending on the mapping style certain values might act completely differently and should be configured to match the desired difficulty. This affects HP and AR the most, because there are times where HP5 might be too much for an insane, and when AR7 might be too little an insane.
Slower and/or calmer songs would most likely warrant lower difficulty settings overall, compared to the faster ones.

Important thing is to use common sense and your own judgement to these values and recommend a change if any of these don't feel right for you.
Just one note about OD, a lot of people prefer lower OD because it's easier to get 300s, however that is not always a good thing. osu! is a rhythm game and the biggest thing that comes to mind when rhythm is concerned is the timing of the notes, so stricter timing is always encouraged, but it should be used within reason.

I didn't include Extra because technically that should be a very hard Insane. In that case just use some common sense, such difficulties should not have OD lower than 8 and AR lower than 9.

There is also one more thing which is relevant to this step, and that is the Difficulty Spread. You can find more on this in the Ranking Criteria.
You should try to judge does the mapset have proper difficulty spread. It is important that pretty much the above 4 (or 3 highest) difficulty categories have a progression which doesn't stand out too much from the each map.
While it is currently rankable to have only 2 maps in a mapset (a normal and a hard map), it is not recommended because there are very few songs which could actually justify this.

Step 5.2 - Tick Rate

Tick Rate can be a difficult subject and is one of the most subjective things in mapping in general. However, even despite being subjective, there is generally a way a person determines which tick rate could fit best and why. This part will explain the Tick Rate more and why should it get modded and why is it sometimes necessary.

If you don't know what Tick Rate is, it's the interval between ticks on a slider, expressed by a number of beats. Having it set to 1 means that a tick will happen on a slider which is located 1 beat away from the slider start and every beat after that until the slider ends (the tick cannot occur on the slider head, arrow or tail). Having it set to 2 means that two ticks will appear for every beat, etc.

Generally, if the BPM is correct, tick rate 1 or 2 will be the most fitting value. If the music is quite lively, 2 is almost always the better choice because it stabilizes the rhythm better.
Though, a lot of people suggest tick rate 2 because they have seen other people doing that, or just outright declining tick rate 2 suggestions because they hate to change things which don't matter to them. Don't do this. If you don't understand why the tick rate should be changed, don't suggest to change it.

Sadly I can't help you understand it, but I can give you a general pointer and that is to listen to the music closely and just notice if there are any distinct instruments playing every 1/2 beats for the most part of the song. If they are, tickrate 2 will probably be better, but that all depends on the way the map is mapped.

Tick Rate 3 is an interesting value. It should generally be used if the whole song is based on a 1/3 timing snap rather than 1/2. Such music is kinda uncommon but it still happens, mostly in swing and jazz (and others). If the song is not based on 1/3 timing snap, you should not use tickrate 3, because it just doesn't work.

Tick Rate 4 doesn't have much use but there are times where it could be used, but those are usually done by a mistake, when people use Tick Rate 2 and doubled BPM. The value 4 itself is almost never used though, but technically it can appear in some maps as Tick Rate 2 due to doubled BPM. In this case it's probably best to suggest to use Tick Rate 1 because the BPM is doubled.

Step - 5.3 Slider speed, multiplier and Slider Velocity

To avoid confusion, while speed and velocity might be the same thing, technically they are not because Slider Velocity (will refer to it as SV from here on) is a value you can set, while speed is the relative speed of sliders.

Usually SV should be set to a value where relative slider speed matches the speed of the approach circles as much as possible. This also affects how spacing multiplier works (making 1.0x~1.2x range the best "flowing" one most likely).
Sadly, SV is very difficult to change because it affects the whole map. A small change will break pretty much every slider on the map and will probably need major adjustments and maybe even a remap. It is still good to give your opinion on it because while it maybe be within the acceptable range, but not optimal, the mapper might receive that advice for their next map!

SV can be changed through a multiplier which is located on an Inherited Timing Section (will refer to it green section from here on) during a map. This affects how fast a slider is, but it's not always the best idea. Such sliders have to fit the music, and more importantly, have to be able to get read normally.
There are various ways of making reading possible, but the best way and currently most popular one is using Tick Rate 2, because most of the time such a tick rate already fits the song. It is also probably best to change a shape of such a slider compared to the other sliders around it, just so it stands out better so it's more noticeable.
It is also most often done with a combo change too (applying a new combo), but this doesn't really make it that much more readable, and such a combo change is pointless if the slider really doesn't fit. The combo change is most usually done for the change of the color rather than a group of objects though.

Step 5.4 - Hitsound Set & Volume Control

There isn't much to write or check in this step, but it's good to know these things are there.
It is important to check, especially in Taiko maps that the hitsound set is always set to normal default (unless custom taiko hitsounds are being used, which is extremely rare and almost never seen).

If something feels too loud or too quiet, be sure to point it out.
Be wary of the 5%~25% volume green sections, in most cases those render hit objects inaudible! Inaudible objects (apart from slider ends in certain situations, and spinners) are not rankable!

Keep in mind that custom sets will most likely be used, and they affect which sound samples are being played. Having custom hitsounds enabled without having custom hitsounds in the mapset folder will not have any effect compared to the default hitsound set.
Sometimes people don't apply the correct hitsound set and forget about it, but that is very easy to spot.

Step 5.5 - Inherited Timing Sections (Green Sections)

There is very little to say about these. I have covered most of the things they affect in the steps above and that is hitsounding, changing volume or a slider multiplier.

Sometimes you have unnecessary green sections on a map, but you do not necessarily need to point those out. This means that they are positioned over empty time line, which means they don't affect any objects.
They don't matter, however if you wish you can point out that they exist, and if the mapper cares about making his map clean, the mapper might remove them.

Step 5.6 - Note Snapping part 2 - Wrongly Snapped Notes

I am listing this here because this isn't always easy to detect.

Wrongly snapped notes are objects (will just refer them to as notes for simplicity reasons) which are snapped, but on the wrong ticks.
When timing changes and a resnap was done, sometimes certain notes will not go to their correct ticks.

The most common situation where it happens is when a song has 1/3 notes but majority of the objects are on 1/2 and 1/4 ticks. Resnapping the notes using the 1/3 Snap Divisor will most likely break all your 1/4 notes, and using 1/4 snap will most likely break all your 1/3 notes.
Sometimes they get snapped correctly after a resnap, but in most cases that is not the case.

Another situation is when the mapper doesn't actually know if the music is playing notes on a 1/4 snap or 1/6 snap (or even 1/8 snap). Or if they are intentionally mapping notes using a different snap.
In this case you have to consider a few things first. While the map should always follow the music, the music also needs to be comprehensible to a person. If you have a huge clusterfuck of high speed notes, the reason is quickly lost and it's hard to determine what is going on in the music.
You can use 25% speed to check this more easily.

However in some cases using a more simple snap (for example 1/4) over a 1/6 snap may be acceptable in Taiko for gameplay purposes.

This is one of those times where song analysis comes in handy yet again. Learning how to analyze songs properly will help you when both mapping and modding, a LOT.

Step 6 - General Design Issues: Grey Area Mapping, Weird Rhythms and Overmapping

This is the last step and is heavily subjective in itself, but that doesn't mean that the above terms are subjective, I will explain.

Step 6.1 - Grey Area Mapping

First, what is Grey Area Mapping?

Grey Area Mapping is a term I personally use to define mapping elements or a style which are not defined to be against the Ranking Criteria, but they are usually close to unacceptable to do and are not even in the Ranking Criteria.

It is also a term which nicely defines elements which require a heavy dose of subjective judgement when it comes to lining up the said object against the Ranking Criteria. These elements cause a lot of discussions and drama and are not recommended to have. Often people will not even like those just because they are so obscure and weird (some people will do, everyone is different), so unless you are 100% sure what you are doing and you can pull arguments from the song itself to justify that way of mapping, try not to do this.

For example, sliders which overlap into themselves are not allowed, however it is possible to shape them in a way they are purely distinguishable, but just barely. This is a really obscure way of mapping and very few people can do this right, and I recommend that you avoid anything that cannot be defined by the ranking criteria. It WILL cause a massive debate which will lead nowhere. In every case, it is possible to make things more simple so they don't reach this level.

This also includes objects which are not readable during play because the map is so confusing. Playing skill applies here, this is why this is such a touchy subject, however you should map for everyone if you are aiming for ranking, not those 5 people who can read or play anything.
While this can bring the aspect of "Challenging", I don't think that's the right way, because you should not have to memorize the map to be able to play it, ever.

The above may seem more of a rant, so I will leave it at this, let's do something about this!

This is a very advanced and difficult thing to notice or to explain and is often ignored in mod posts and then you have situations where nobody says anything for many mods and then a staff member mods the map and suddenly you have walls of text.

When you testplay a map, note all things which are very difficult to read. This assumes you are able to play the said map, but all advice should be welcome on this subject. If things bother you, you really shouldn't be afraid to point them out, you just have to say why they bother you, and maybe even provide an alternative on how to solve the issue (the basics of modding!).

Step 6.2 - Weird Rhythms

Another one of very subjective issues, but these can be explained quite easily.

Does a certain rhythm bother you? Go check how it sounds in the song, if it's there, it's probably alright to leave it, but if it's not, it should definitely get changed. That is, if the rhythm that is mapped makes absolutely no sense compared to the music (which is why it's such a subjective issue).

If there is something which is located on, lets say for example the third 1/4 tick in the music of a measure and 1/2 was mapped, essentially the original rhythm gets stomped over and feels wrong.

This also includes even really simple rhythms such as triples. A lot of people just map these and claim them as their style, and that is not that wrong, however if they do feel wrong, be sure to point it out. Often people might decline suggestions which would make them remove these, but you should try regardless if it bothers you.

Step 6.3 - Overmapping

Overmapping is the the mother of all ambiguous discussions and while it's clearly defined, the usage of it is not.
This will not be an explanation of when it's okay or not to do it, let's keep it simple.

Overmapping is usually done to increase the difficulty of a map or to make rhythms more intense. While this is somewhat acceptable today, the usage just cannot meet the common ground between different people.

It is not that difficult to notice, because the most common type of those things are (random) triples placed around the map, and other types of streams. If they feel wrong to you, always check if they are in the music. If they are, chances are the mapper didn't design those notes right, probably (and that means that they are not overmapped actually), however if they aren't simply provide reasoning why are they bothering you.
Notes should not bother you if they are not always in the music, but if they ruin the atmosphere for you, or anything that you might find wrong when you are "feeling" them, feel free to point them out. (this actually goes for anything, modding 101!)

I also have to suggest that you should probably not give suggestions that a person should overmap when originally it did not, because it's not always the best thing to do even if you personally think it is. You have to keep this in mind when you are suggesting that a mapper should add a note somewhere, there has to be a reason why the note should be added, backed up by the music itself.

Step 6.4 - Map Design - How does the map look?

This is the final step. While it's quite easy and probably the most common step in everyone's modding, it's the least important one, but it's still important to an extent.

While looks are important so the map is pleasant for many people to play, the looks may not define the quality of the map itself. Sometimes people get lazy and just want to speed up their mapping process so they start making more shabby slider shapes, patterns and just use a more random way of approach to mapping.
Many consider mapping an art, and you can have your approach towards it, but you should always consider the playability. Maps are meant to be played, not watched. At least those maps that are aiming for ranking. So even if the map looks extremely good, if it plays really poorly it has a flawed design.

Nazi modding covers this step almost entirely as well. While it is discouraged to provide nazi mods, it is still useful to point out things that just look awful or distasteful, such as really weird uneven patterns or slider shapes (sometimes when people aim for symmetry, they mess it up a little so it's off as well).
A lot of times people don't even bother with these, and a lot of mappers will not even change things based on this, because they think it's not important. You can't really blame them, but every mapper who cares about their work and didn't notice small things may appreciate this.
Just be prepared for that the mapper will most likely decline most, if not all nazi mods.

Taiko additions and differences to the above guide

You have to know that HP works different in Taiko. Instead of having a bar that drains over time, you will have a bar which fills up if you hit a note, and which will drain if you miss a note. Upon finishing of playing a Taiko map, if you have 50% or more HP left, you pass the map, else you fail.
Longer maps are harder to pass than shorter maps, with the same HP setting, because the drain is much harsher on longer maps, because there is more notes.

Approach Rate works differently too, so does the Circle Size. They actually don't affect Taiko in any way on a gameplay level. Though, according to the standards, those two values should always be set to 5.

Sliders don't have a click -> hold -> release type of gameplay. They become drumrolls which can be hit by the player how the player wishes. Missing a slider will not make you miss or lose your combo.

Spinners work differently too, because spinners are not of fixed length, but are determined by the amount of hits based on the length you chose. This means that people who click very fast can complete extremely long spinners very fast. They also don't break your combo or cause you to miss if you don't complete them.
Also it is very important to notice that spinners do not create a hit when they end. A lot of people choose to end spinners on a downbeat in standard mode and apply a heavy hitsound to it, but that will not work well in taiko. If a player finishes the spinner early (and in 99% cases people who care to spin fast do, else they just don't finish it completely) there will be no note on a downbeat!

Important Notes

First of all, the Ranking Criteria main page does not include Taiko in it. For that you need to navigate to the Taiko sub-section of the Criteria which can be found here. It is important that you are familiar with it, because some technical things work completely differently compared to standard. (this can work for other modes too, they have their own sub-sections too)

You don't have to have almost any Taiko knowledge to be able to provide a useful mod. Taiko maps can still have wrong General or Metadata values, and they can also suffer from various issues any standard map can suffer from. Even a person who is completely clueless about the gameplay mechanics of the mode can still find obvious errors. It is important that you try, don't skip Taiko maps just because you can't play them or mod them on a gameplay level. This actually goes for any game mode, just check things you are able to check.

It is also very possible to have wrongly snapped notes in Taiko maps, however they are a bit harder to find because of how Taiko works (with all the rhythms and stuff). If you don't have experience with Taiko, you probably shouldn't waste your time on this, and just provide the information about general errors you may find on the way.

As for the note structure, if you don't understand how Taiko works, you most likely won't be able to make much sense in it, or even point out why are some things wrong. You need to understand how the mode works and how the map plays in order to be able to properly mod the note structure in depth, and this guide will not cover that, since it requires experience which you cannot be taught by someone through words, you need to be able to understand and maybe even play the game mode.
But don't let this discourage you, if the map sounds too intense when the song is calm, that is already a good find and you should probably point it out.