Say you are making a belt, 1 1/2" wide, I use a clear plastic ruler ( the ones you can buy at ( Wal-Mart ) Lay the ruler on the belt, line up the edge of the belt with the 3/4 inch line on the ruler, take a pen or any pointed object an put a mark every inch. Most belts have 5 holes. When you punch the holes, use your eyes to center the punch.
I hope this helps you.
Dan