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.
Step 2: Re-attach the Ends
If your belt is leather, as many are, remember to use a leather needle, and do not make too many stitching lines! Each stitch makes a hole in the leather, and if you make too many stitching holes all in a line, it becomes perforated and may tear. You have to strike a balance between enough thread to hold it securely, and not too many holes to weaken it overmuch. A large stitch length is also recommended.
The belt will now fit properly. It does look altered when off, but when worn, the seam is hidden by the tail coming around through the buckle!