To adjust the length of your belt:
1/ Place the belt around your waist and close it.
2/ Release the strap by opening the clasp behind the buckle.
3/ Make the strap slide until it fits how you want it.
4/ Close the clasp behind the buckle to fasten the strap.
5/ Use quality fabric scissors to cut the surplus of strap.
Beware not to cut your belt too short, as the operation is irreversible. The drawn instruction manual here attached may also help you visualize the operation.