How do you create a custom boss on a SNES rom?

I found this rather interesting boss fight from FF6 a while ago:

YouTube - Final fantasy VI boss hack: Apple

How do you make stuff like this? :eek:
I wanna create my own bosses as well!

There are many different utilities that allow you to do this. Some easier than others. Just search something like "Rom Hacking Utilities" on the internet.

Yeah, what you need are ROM hacking communities. It's not easy though, you have to have some programming skills.