As it said in the subject
Please help.

I was actually planning into learning to mod Mania maps as well, especially because I'd like to see more 8K maps ranked and the easiest way to help do that is to mod them.
At first, I was kind of unsure as to how to do it, too, but in the end I figured out that it's pretty much the same as modding osu! maps. You look at the map in question, play it a few times, watch it play in the editor, and note down things that seem odd or just don't fit.

When modding (or improving my own maps), I usually start out with pitch relevancy, something that Hanyuu explained in his mapping tutorial pretty well. Basically, after playing the map once or twice, I let it run through the editor at 50% speed and pay close attention to whether the note progressions in the map fit the note progressions in the song. Depending on your prior music knowledge, this might be easier or harder to do, but there's an easy trick: If you want to see if note 2 is higher or lower than note 1 on the chart, just whistle note 1 and then note 2. If the 2nd note is higher than the 1st, but the beatmap notes go from the 5th key to the 4th, you can see something is wrong and note that down in your modding post.
While checking for pitch relevancy, you can also easily look out for "ghost notes", which are notes that don't follow anything within the song itself. Often, they're justified because they're meant to create tension or add to the rhythm, but sometimes, they're misplaced or just shouldn't be there in the first place.

The second thing I usually look at is Bracketing, something also explained in the tutorial. Very very often, new mappers (myself included) tend to not make a clear distinction between the instruments their beatmap is following, which results in patterns that are unnecessarily hard to read for the player. Probably the worst offender of this is putting the drums on the 7th key in 8K mania (or the center key in 7K), which can make sense if it's visually distinguishable from the melody (i.e. the melody mostly takes place between keys 1 and 5 in 8K mania, or you're putting a clear pattern "around" the center key in 7K mania), but often just seems "lumped in", to the point where the player subconsciously asks themselves: "Why is there a chord here?", because it's easy to mistake a drum pattern on the highest key for the top note of a chord.
Basically, the idea here is to check if the mapper is mapping two or more different instruments at the same time, and if he is, how clearly distinguished these instruments are from one another. For a really good example of this, see DJPop's Hades in the Heaven maps, where he represents the synth with holds and the drums with regular notes. Another good example is richardfeder's Phantasmagoria 7K HD, which has the vocals mapped (mostly) to long notes, with the drums interspersed in a very logical and fun fashion. Another way to map drums in an intuitive way is to map them as "chords", with a little pause behind them, giving the player the visual clue that there's going to be somewhat of a stop within the melody, which they can then interpret as a likely place for a drum sound to be.

And finally, you should check for beatmap integrity. This is something pretty abstract and I don't know much about it myself, but there's certain ways you can make maps feel "heavier" or "lighter" (the most common way would be through note density, but there's other ways, too). Besides the obvious timing questions and whatnot, beatmap integrity deals with how well placed those heavier and lighter sections are. For example, trills feel heavier than non-spam stairs do, for the simple reason that trills create more tension for the player('s fingers). So, let's say, if you map fast trills in a section where they might fit to the music, but don't fit to the overall "feeling" of that section, you might still be doing something wrong, or at least odd, even though you followed the music correctly. Stuff like this is also a matter of dominating vs recessive instruments within the song, and at this point, we're almost into music theory...
Basically, you should be able to tell if something feels really off, and if that's a part that's otherwise correctly mapped to the melody and rhythm, chances are it has something to do with beatmap integrity. For a map that really gets this message of "feeling is important" across, try Entozer's ChaiN De/structioN, which is probably one of the most well-mapped "easy" o!m mapsets out there.

Of course, there's a myriad of other things to look for as well. There's the obvious stuff such as correct timing, adherence to ranking rules, etc., and the more difficult stuff like speed changes or keysounding. For the latter two things, you're going to have to ask someone else, though, because those are pretty complicated techniques that only a few people actively use (then again, they can be so amazing if used correctly).

Aplause to Aposke since you pretty much pointed it down as specific as it needs to be.
I share the same idea and i think alot of mappers and even admins are way too laid back on such terms.

Aposke wrote:

I was actually planning into learning to mod Mania maps as well, especially because I'd like to see more 8K maps ranked and the easiest way to help do that is to mod them.
At first, I was kind of unsure as to how to do it, too, but in the end I figured out that it's pretty much the same as modding osu! maps. You look at the map in question, play it a few times, watch it play in the editor, and note down things that seem odd or just don't fit.

When modding (or improving my own maps), I usually start out with pitch relevancy, something that Hanyuu explained in his mapping tutorial pretty well. Basically, after playing the map once or twice, I let it run through the editor at 50% speed and pay close attention to whether the note progressions in the map fit the note progressions in the song. Depending on your prior music knowledge, this might be easier or harder to do, but there's an easy trick: If you want to see if note 2 is higher or lower than note 1 on the chart, just whistle note 1 and then note 2. If the 2nd note is higher than the 1st, but the beatmap notes go from the 5th key to the 4th, you can see something is wrong and note that down in your modding post.
While checking for pitch relevancy, you can also easily look out for "ghost notes", which are notes that don't follow anything within the song itself. Often, they're justified because they're meant to create tension or add to the rhythm, but sometimes, they're misplaced or just shouldn't be there in the first place.

The second thing I usually look at is Bracketing, something also explained in the tutorial. Very very often, new mappers (myself included) tend to not make a clear distinction between the instruments their beatmap is following, which results in patterns that are unnecessarily hard to read for the player. Probably the worst offender of this is putting the drums on the 7th key in 8K mania (or the center key in 7K), which can make sense if it's visually distinguishable from the melody (i.e. the melody mostly takes place between keys 1 and 5 in 8K mania, or you're putting a clear pattern "around" the center key in 7K mania), but often just seems "lumped in", to the point where the player subconsciously asks themselves: "Why is there a chord here?", because it's easy to mistake a drum pattern on the highest key for the top note of a chord.
Basically, the idea here is to check if the mapper is mapping two or more different instruments at the same time, and if he is, how clearly distinguished these instruments are from one another. For a really good example of this, see DJPop's Hades in the Heaven maps, where he represents the synth with holds and the drums with regular notes. Another good example is richardfeder's Phantasmagoria 7K HD, which has the vocals mapped (mostly) to long notes, with the drums interspersed in a very logical and fun fashion. Another way to map drums in an intuitive way is to map them as "chords", with a little pause behind them, giving the player the visual clue that there's going to be somewhat of a stop within the melody, which they can then interpret as a likely place for a drum sound to be.

And finally, you should check for beatmap integrity. This is something pretty abstract and I don't know much about it myself, but there's certain ways you can make maps feel "heavier" or "lighter" (the most common way would be through note density, but there's other ways, too). Besides the obvious timing questions and whatnot, beatmap integrity deals with how well placed those heavier and lighter sections are. For example, trills feel heavier than non-spam stairs do, for the simple reason that trills create more tension for the player('s fingers). So, let's say, if you map fast trills in a section where they might fit to the music, but don't fit to the overall "feeling" of that section, you might still be doing something wrong, or at least odd, even though you followed the music correctly. Stuff like this is also a matter of dominating vs recessive instruments within the song, and at this point, we're almost into music theory...
Basically, you should be able to tell if something feels really off, and if that's a part that's otherwise correctly mapped to the melody and rhythm, chances are it has something to do with beatmap integrity. For a map that really gets this message of "feeling is important" across, try Entozer's ChaiN De/structioN, which is probably one of the most well-mapped "easy" o!m mapsets out there.

Of course, there's a myriad of other things to look for as well. There's the obvious stuff such as correct timing, adherence to ranking rules, etc., and the more difficult stuff like speed changes or keysounding. For the latter two things, you're going to have to ask someone else, though, because those are pretty complicated techniques that only a few people actively use (then again, they can be so amazing if used correctly).

Thanks Aposke

Oh well i was thinking about this thread again and notecharting in general and i just want to write something down. It is allready been mentioned a few times but pitch relevancy is probably the most important and required thing to keep in mind when mapping or modding (for me atleast). Everyone knows how it looks and sounds like if someone wipes over the piano keys from one side to the other one and and the most basic and simple notepattern for that in the game is just the same thing. Notes from one side to the other one evenly, ascending in pitch from the left to the right side. Again thats really the most basic thing and playing only notes like those get boring quickly but it is just important to keep that whole ascending/descending thing in mind. There is really no way to bring every note placement exactly as it is over to the game, well since its just a game but if the notes progress just roughly or even very similar thats just good enough. It makes it all just very natural to play and you can sort of anticipate in what regions of the beatmap the notes will fall. Well

For things to look out in a map is if there are notes in just really obvious wrong order. For example a short stream of notes that go from left to right while it is clearly hearable and understandable that it should be exact opposite way. Something like that should allways be corrected in my opinion.
I just want to add at this point that when i started playing and mapping there was this reverse pitch relevancy thing. I dont really remember why and how that came up but it is just wrong and seems like someone just made it up, its just weird.

Thats not that difficult to do for the more normal up to hard diffs. As the difficulty gets higher and more notes come at the same time, its good if the note layers seem understandable and if you can make out the seperate layers. Then you can just check one layer of notes and then the other. Since the maps are usually from mp3 files its important that they atleast seem like the creator had some idea in mind while mapping and the map feels fitting and intuitive when playing. If something feels a bit off or just odd its okay to mention atleast that, even if you dont know exactly what it is. Sometimes it gets difficult to understand every note and the mapper should know the reason why they put the notes the best and maybe finds something when looking at it again. Well any feedback is better than none