This is a pretty simple mod, do you have a tri-wing screwdriver? If so, all you need to do is unscrew the six screws on the back of the GBC, take off the back of the casing on the GBC (it'll just slide right off) unscrew the 2 or 3 phillips head screws on the mobo (motherboard), lift the mobo, and viola, the buttons are right underneath. Then you can swap the buttons out with whatever you want! Just make sure what you want is the right size, as for an NES controller's buttons, I'm not sure. You may need to trim them.
So yeah, it's pretty straightforward, a simple beginner's customization. (I can post pics if necessary)
