Place the buckle through the middle hole of the belt.
Wrap the belt around your waist to get the appropriate length. Make a small mark on the inside so you know where to cut. Leave some length to spare as it is easier to trim more if necessary.
Cut the belt with scissors on the square end, not the end with the buckle holes.
Insert the cut end of the belt into the buckle and clamp it down.
Undo the buckle and try the belt on for size. If any trimming is needed redo steps 1 - 4