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.
Now making a new design for the skin. Take a piece of paper and trace the original skin, button holes and all. Now for the tricky part. Cut it out, making sure you have enough room for the buttons, or else the buttons won't push. Glue or draw your image on the traced skin. Look on the last picture for a (horrible) template.
Step 3: See if it fits
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 fit. Now you can play in style!