You should replicate the song instrumentation in a way that it doesn't blend in too much (with the actual music) for feedback reasons but also doesn't dissonates from the song original percussion and melody.
Hitsounding can be split in 2 main parts:
Instrumental is the most important part in terms of feedback. Usually accounts for the majority of the samples.
Melody is for string, vocals, piano activelly mapped that also needs feedback but can't be hitsounded as beats only. (For this tutorial keysounding won't be mentioned as a part of melody hitsounding.)
Almost every sample available will be used for instrumental, leaving only normal-hitwhistle and soft-hitwhistle for melody samples.
First, you need to know what sounds are used for most music in its percussion. For the sake of simplification, we will use rock. For a drum-kit that makes most of the percussion in rock music you need to know each of the following sounds;
There are more, but with this ones you can pretty much hitsound most music out there. To learn each of this sounds I recommend the following youtube video;
For the sake of simplification, I'll be forcing a name choice for each sound in the folder, but this doesn't mean you can't find your own way of doing.
Sometimes a sample works in some sections of the song, but in others it doesn't (like calm vs exciting parts). For that purpose you can either use the Snare 1 and Snare 2 or you can change the sampleset.
Another thing worth mentioning is that the passive hitsounds are sounds that every object will have when you set which sampleset you're using.
Example;
Having the sampleset as "Soft" Custom 2 will make all the objects starting on that greenline have the passive hitsound of "soft-hitnormal2" if you have that in your folder or the osu! default "soft-hitnormal". Same happens with Drum and Normal, but for the sake of this tutorial you will mostly use Soft for passive hitsounds.
In the mapset you will find many songs that were hitsounded in 1 or 2 specific sounds. The goal here is that you learn the nature of each sound used in a song to identify its nature.
Make sure to check the difficulties and also other maps that you might like to figure what drum sounds and melodies are being hitsounded.
Once you understand the basics of what's a cymbal finish, snare, kick, tom you can already start to hitsounds most regular songs. The next step is picking samples in the folder accordingly to the song.
Now that you have all the samples with the correct name in the folder you need to place the hitsounds in the map, checking samplesets, how the hitsounds sounds with the music in the editor to balance volumes, offsets, check the channels (right and left).
There are two ways of hitsounding:
The first way you make that the difficulty is balanced and every object is correctly hitsounded. The second method allows you to copy the hitsound to more difficulties without any concern of missing any sounds that were not mapped in a difficulty among others.
In case you're starting I recommend trying the second method because that lets your focus purely on the hitsounding and you can also use the background as a way to visually see the hitsounding and improve.
Make sure to have in your computer a hitsound copier tool in case you make a hitsounds difficulty so that you can apply it to the entire mapset.
My personal recommendation is the Hitsound Copier by Olibomby and Potoofu
This is very simplified and generic. Just like mapping, hitsounding allows you to be creative in many ways as well, so make sure to learn the basic and don't forget to keep trying doing the best you can.
Good luck with your hitsounds.
Best Regards,
Net0.