In order to attach the belt to the guitar, we'll need plain leather on both ends to slip onto the strap buttons. We also want to be able to adjust the length of the strap, so we need to keep the buckle intact. Luckily, this can all be accomplished with one simple cut.
Cut off the non-buckle end of the belt, leaving at least 1" of plain unstudded leather on the long piece of the belt. If you don't like the squared-off ends this leaves behind, you can angle-cut or round off the cut ends. Just make sure that there's enough leather left to make the holes for the buttons in the next step.