You want to make three cuts:
2 on the back to loop the belt through
1 on the side, whichever side you put the rest of the belt to
So crack out your Dremel
with the cutting tool and get working.
If you really feel the need for perfection, or if you need a certain belt size, you can measure it out and draw guide lines on the back prior to cutting. But be careful --- Dremels are dangerous and watch out for sparks. Wear eye gear and gloves when using the Dremel.
After the cuts are made, put on the sanding head and smooth out those cuts. You don't want to ruin your belt on a jagged edge or cut yourself.