there is a lot to cover here, to make things easier there are stage managers and costume managers.
In brief:
Costumes go into the fighters folder on your SD card. fitSamusXX eg.
The XX represents the costume slot. look up the pallet swap for brawl to see which number reps which colour for each character.
if you want to update the images on the CSS, RS and BP (character select screen, ersults screen and battle portrait) to new ones you have to go into other areas. and use brawlbox
info/portrite has the portraits for BP, jsut open them, find the texture, replace it with the one you have (make sure file sizes arent too big or youll crash)
menu/common/ gets you to character busts for RS pics, again, replace with your new one.
open common5 under system folder for the CSS. youll have to flick through the trees and nodes to get to the right place.