Step 1: Measure and cut
Cut the belt near the buckle, making sure to leave a half inch (or so) of the single layer of belt material after the folded over part (see pic). Cut the body of the belt again, your calculated distance from the cut end. You now have three belt pieces: the buckle piece, the too-long piece (discard this), and the tail end. The buckle and tail pieces together are about half an inch longer than you want the belt to be.