First you plan, then you come up with ideas related to the plan, next you plan some more, pick up more ideas, ..., until you feel like you have gathered enough stuff to make a rom hack based on that planning.
What comes to hacking process itself, you could take a look at my video tutorials in YouTube if you like. They show you how GameBoy rom hacking works (which is more difficult than doing stuff with GBA games due to things called pointers). I could personally hack any gba game no problem so if you wish to hack FireRed, Emerald or something, you'd be ready for that too.
I don't have time to teach anyone personally though.