How would I get started?

Gi Nattak pretty much nailed it all, but modern FF3us hack must have something that makes them stick out the the batch.

Yo can make a "hack" or mod only with the editors available, but it won't give you mod a unique feel.

I suggest after getting familar with the editors that you learn Hex. Then you could learn how to modify events or even create your own. New music and sprite work can also help having an original hack. You can finally open a world of possibilities by learning the 65816 assembly language. Various tutorials for this are available on the internet.