Basically, you should look for a program called Nightmare. This allows you to edit a Fire Emblem rom.
Then, you look for different Modules, which allow you to edit specific parts of a rom: i.e. chapter maps, classes, characters, etc.
You should also look for a text editor while you're at it.
I'm doing the same, but for Fe8...
it fails. D:
With some tutorials, you just get the 'hang' of it, you know? :D