Modding is the process of examining another's map and making suggestions. These suggestions can be either to fix errors or to simply improve the map. The person modding the map is called a "modder", and the person who made the map is referred to as the "mapper". Modding can be useful in three main ways:
If you are learning how to map or already know how to map, modding can be a great way to improve mapping skills.
First, download a map from the beatmap thread, which can be located in either the Pending or Work In Progress/Help forum (not the Ranked/Approved Beatmaps forum!). You can either randomly choose maps from these sub-forums to mod or you can open a modding queue in the Modding Queue sub-forum under the Work In Progress/Help forum and accept requests. If you are opening a queue, it is a good idea to see how these queues work by reading some threads in the 'Modding Queues' sub-forum.
Choose any of the difficulties to begin your mod. It is recommended to start with the easiest or hardest difficulty and make your way up or down the different difficulties.
Start by playing this difficulty in either Play mode or test mode. Play-testing is an important part of modding as it allows the modder to understand whether or not certain patterns play well. Take note of anything that felt odd.
After play-testing, watch the song in the editor and pause frequently to catch any errors. In this stage of modding, it is most often that modders find more intricate problems such as combo errors, hitsound inconsistencies, and areas where hit circles or sliders are missing. When you find a hit circle or a slider that needs attention, select it. Now, go to the "Edit" tab and select "Copy" at the top (or Ctrl+C), and then to your post you started up, and paste (Ctrl+V) what you just copied there. Then, follow up with either what you think is wrong (Ex. "I would change this spacing because the music does not suggest a jump here), or simply that it doesn't seem right and you can't put your finger on it. Always look for things that you didn't notice while playing.
Rinse and repeat with the next difficulty keep in mind that if you see something while in the Editor that you didn't notice while playing, point that out too. (It is recommended to have the map play at normal speed in the Editor while you watch it, just to double check things). When you're done, submit your post.
A very helpful tool is AIMod, which can be opened under the "Files" tab or by pressing Ctrl+Shift+A. This shows the errors that the certain difficulty of a beatmap has. However, be wary that it is still just a computer, and some errors are not true errors (i.e. a spacing error, but there was supposed to be a jump).
The below is a list, organized by importance.
The most important part of making a beatmap is timing. If the BPM is incorrect, the whole song will be timed incorrectly, and the rhythm of the notes will feel awkward. If the notes feel consistently "off," then it is most likely due to an offset issue. The offset adjusts the main beat of each measure so that it will match the main beat the music. This is likely the problem if it is hard to get good accuracy on a map. The BPM and offset must be the same in all difficulties. This also applies for BPM changes. All BPM changes and their correlating offset must be the same in all difficulties.
One of the most important aspects of beatmapping is spacing. Spacing helps the player understand the rhythm of the hitcircles better. Spacing rules can be easily broken while still having a easily readable beatmap, but doing so requires a higher level of mapping understanding. If you are a beginner modder/mapper, don't hesitate to comment on spacing, but also be weary that weird spacing can be characteristic of a jump, an anti-jump, or a jumpy map in general.
Being that osu! is a rhythm game, sensible rhythm is very important in beatmapping. If certain notes seem to be missing that may make beatmap "flow better" or sound better, then do not hesitate to suggest doing so. If certain notes seemed to be overmapped, then do not hesitate to suggest removing it. If any sort of rhythm sounds or feels awkward while playing or while looking in editor, then it is likely that the hitobject may have been wrongly placed, wrongly timed, or given too short or too long of a length (e.g. sliders or spinners)
Combos in the most basic sense divide the hit circles of a beatmap so that they are easier to read. Combos generally do not go over 20 in Insane difficulties, 12 in Hard difficulties, and 8 in Easy and Normal difficulties. There is no problem if these combos reach above the general rule if they make sense, but if there is a logical way to break up a long combo, then it is recommended to do so. Combos are usually placed to either match the vocal/musical phrases or to indicate the downbeat of each measure. In special cases, combos are used in indicating any discrepancies in spacing or slider velocity.
Overlaps and stacks are generally fine. However, sometimes these overlaps and stacks are repeated to an extreme extent which makes maps very hard to play. Anything hard to read because of weird shapes, overlaps, or stacks should generally be avoided and is a good thing to comment about in a mod post.
Sometimes maps contain slider velocity changes that may not make much sense or are very hard to follow. If a slider velocity change requires more than just intuition to understand, then it is usually not a fitting slider velocity change. A common example of when to use a slider velocity change would be in a section of a song that is slowed down or more calming than the rest of the song. Here, many mappers would likely slow down the slider velocity to match the calmed music.
Hitsounds are the noise the game makes when the player successfully hits a note. All maps must have a sufficient amount of hitsounding, whether using custom or default hitsounds, or else they cannot be ranked. Sometimes the hitsounds can sound awkward or weird. Other times, the hitsounds are too quiet or too loud. This may actually confuse the players. Something to be very careful about is using too low of a volume. In most cases, using a volume that cannot be heard is unrankable and is something to pay attention to.
Kiai Time should make sense and should not be abused. They are only for times where the music reaches some "epic" climax. In addition, Kiai Time should generally be the same in all difficulties. However, Taiko difficulties may differ from regular osu! standard difficulties. If the Kiai Time differ in any way, it is not a bad idea to make sure whether the Kiai Time are different on purpose or by accident.
The difficulty settings can be found in the Song Set-up window, opened by click F4 or the "Song Set-up" tab at the top of the editor screen. These are important to game play so that the song is at a fair level of difficulty for the varying difficulties a mapset can have. For example, an Easy difficulty will not need an Approach Rate (AR) of 7 or an HP Drain Rate (HP) of 8. This is completely illogical and makes the difficulty much harder than it should be.
The actual Song Setup options do not impact game play, but it is important to give correct names. The artist name, the title of the song, the source (if applicable), and the tags should all be the same in each difficulty. Audio Lead-in times are usually the same, but can differ depending on when the first note is played. The "Enable Countdown" button should be either activated or deactivated in all difficulties.
Most importantly, all beatmaps must have a background picture. The skin should not blend in with the background or greatly hinder game play (because of complex hitcircle design, huge hit bursts, etc.). There are also many Skinning rules which are needed to be followed. Similar to Skinning, combo colors should not blend in with the background either. These combo colors should also be very distinct and clear. The mapper should not have two almost identical combo colors (unless they are separated by a very contrasting color in between).
If anything weird in the storyboard happens (bad rendering of pictures, emptiness, etc.) it is a good idea to tell the storyboarder (who may or may not be the mapper) so that he or she can fix it. The Storyboard generally should not exceed an S.B. Load of 5.0x. You can check out the Storyboard by pressing F2 or clicking the "Design" tab at the top of the screen in editor. The dimensions of the elements should not exceed the default size.
All maps must have a preview point. A preview point will be denoted by a yellow-ish line on the timeline at the bottom of the editor. You can also check if a map has a preview point by going into AIMod (Ctrl+Shift+A) and checking under the "Style" tab. This preview point must also be the same in all difficulties.
The bit rate of the song should lie between 128 to 192 kbps. Also, one empty .osb may exist but they must be deleted when more than one is present. Check if there are any file unrelated to the beatmap itself (excluding the thumb.db as this is the metafile automatically generated by the computer system).
Offensive Language Even if the mapper is a close friend, please try to keep the mod post clean. It is understandable that certain patterns may seem completely illogical to you, but that is not a reason for insulting the mapper. Refraining from terms such as "WTF is this?" or "This is completely stupid" can help to repress any potential argument and create a better osu! environment.
Essentially, each point of your mod post should have a "when", "what", and "why". When you do not have the "why" or the "what", the suggestions may be a little confusing for the mapper. This, in turn, makes your mod a lot less useful than it should be. Simply explaining your suggestions will make your mod a lot more useful.