Test it out by applying the skin and pressing the buttons. If they feel good, then you're done. But you may have to trim around the buttons to make it...
I got Guitar Hero On Tour (Nintendo DS) a few days ago, and I made this skin. Here's how to make your your very own GH On Tour custom Guitar Grip (called GG in this) skin!
Step 1: Take original skin out
To take the original skin out, you must first prepare the GG for opening. Fold the strap back and take the pick stylus out (picture 1). Open it by prying the plastic clear part away from the black part by the pick stylus holder on both sides (picture 2) and take the original skin out.