Here we have a compilation of usefull links related to mapping, skinning, etc.

Mapping: 213y2u

[Tutorial] osu!mania mapping, Keysounding by Hanyuu
Keysound copier (Alpha alpha alpha version) by Roxas
[Tutorial] osu!mania mapping, Basics by Hanyuu (outdated)
SV Changes Guide by Agka
osu!mania Mapper List by Maiz94 (outdated, might change this one with a new one)
osu!mania Hitsound/Keysound Copier by [AyanoTatemaya]
[Tutorial] osu!mania Mapping Tutorial by Feerum

Gameplay: 5i1853

[GUIDE] How to improve in osu!mania by Drace
mania [x]k.ini function Manual by dekunuts related with skinning
osu!mania Gameplay Videos! by theowest
[WIP] Pattern Based Mapsets For Training [10/6/15] by abraker
osu!mania converts/packs collection by aitor98

Others: 474z60

osu!mania General FAQ by Evening
[WIP] 4K Skill Analysis Mapset - 4SAM - 1.21v by LeiN-
osu!mania 4K/7K Dan (段) Ranking by Evening
[osu!mania] BNG / QAT List by Baraatje123
Performance Points and suggestions (osu!mania) by Tom94
osu!mania General Discussion by juankristal

If you know more stuff to add here be sure to poke me in-game !