I took a shot at making custom Guitar Hero songs once, I lost interest after about 2 minutes, lol. Lots of steps involved, like having a modded PS2 (which I already had), inserting the new songs, editing titles, reburning the game, ect.
Although they finally figured out how to do it for GHIII for PC, so I may try that again. Seems like it is probably a lot simpler to inject your own mods into the PC version of anything.