Put the GTF pad against the butt, trace a perimeter line with pencil, chalk or something else visible. You're going to want to grind close to the line. If you have a belt-style sander, use it to take most of the "meat" off. Pay close attention to the toe angle. You can put it on the buttstock that's been wrapped with masking tape (I like to use electrical tape) and sand closer, either by hand, or if you're brave, with the power sander. It's going to take a bit without the proper set-up, but it's doable.