This is how you can adjust the belt buckle easily and mount the buckle: 

Remove the buckle fixing screw.

Measure yourself exactly where you will wear the belt.

Subtract the result from the length of the belt.

The result of this subtraction is the length of the piece that needs to be cut off the side where the buckle is mounted.

Make the new buckle attachment hole 1 inch (2.5 cm) from the edge of the belt

Mount the belt buckle - that's it!