Wrap the belt around your wrist and leave an overlap of two inches.
Cut this length with scissors or any other cutting tool.
Wrap the leather around your wrist again and find a comfortable place for snaps to be.
Punch or drill a hole through the leather overlap where you would like for snaps to be.
You can buy snaps at any craft store. Hammer a snap through your hole you've drilled or punches. You might have to wiggle it around a bit for it to fit just right, but you should be able to get it wedged in there.
That's it! Have fun. This is a great and stylish way to reuse that leather belt instead of throwing it out.