The easy way to shorten a belt is just to cut it off at the end with the holes, but what if you like the tail as it is? With most belts, you can also shorten them from the buckle end, preserving the finish of the tail. It's only very slightly more work.
Step 1: Measure and cut
Calculate how much shorter the belt needs to be by wrapping it around your waist where you like to wear it, and measuring from the hole you want to use to the bar the tongue is attached to. In this technique, you will overlap the cut ends by about half an inch (or more if you want), so subtract this amount from the measured distance to get the actual length to remove.
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.