Sooner or later, you're gonna have to do it. It may be because you lost a little weight, and now it's time to notch over one more, and you're plumb out of holes. Maybe it'll be due to the fact that different pairs of pants sit on your body at different places. Or it may be that you'll simply had that belt for a little while, and the leather has stretched a bit.
But, at some point, you're gonna have to punch a hole in a belt. And if you do it right, it can look perfectly in line with the others, like it's been there the entire time.
No matter which of the techniques below you opt for, 90% of your success will be determined not by how the hole gets there, but by where you put it in the first place. In most men's belts, the holes are spaced at a perfect one inch, making your job plenty easy. Simply use a ruler to set up a horizontal straight edge by making sure it crosses each hole at the same point, and mark on the back.
Or, if you're crazy obsessive (I am), you can make a little template with some painters tape. Just use the existing holes to mark their placement, then shift the whole thing down by one. Hold it up to the light to make sure your dots are placed right in the center.
Option #1: The Awl (or Nail) If you have at least a medium-ly equipped tool box, you'll definitely have a hammer or mallet. And you may have an awl, which is (basically) a metal point with a handle. It's similar to an ice pick, which you could just as easily use...if you actually still have an ice pick in 2012. If you don't have an awl, get one! They're like $2.00, and you'll find uses for them in all sorts of wood, paper, leather, and fabric projects. Or, try a large nail.
There's not much to it: Mark your spot, and place your belt on some scrap wood. Use short firm strokes to slowly insert the sharp point into the material, driving it all the way through until you've reached the part of the metal that corresponds to the size of your holes. Which is another argument for the awl over the nail, as the tool is evenly tapered along its length.
Option #2: The Electric Drill If you take your time, and start the hole well, you can drill through leather with fairly clean results.
The trick will be to make the right sized hole, so test by inserting the solid ends of your drill bits until you've selected just the right one. You'll have the best results if you can start the hole cleanly. If you have brad-point bits, you'll definitely want to opt for those over metal tapping bits. If not, make a significant dimple where you want your hole with a nail or sharp knife, and use that to keep the bit in place.
Option #3: The Leather Hole Punch This specialized tool is by far the most ideal option. You can find them easily at any craft store, and they're always less than $10. I found this one at JoAnn's for $8.00, on sale with 40% off.
These revolving punch pliers have a wheel with multiple sizes to punch round holes in pretty thick material. The tension springs make it easy to punch by hand. Seriously for $5.00...this is good investment to have around.
If your belt has oval shape or elongated holes, I'm guess you could use this tool to create the two round corners and then cut out the middle with an craft knife. But even I'm not that obsessive. A well-space round hole will do you just fine.