It's easy to put a belt buckle on a cowboy belt.
Your cowboy belt buckle will have a metal tab on it where the belt can loop through on the back. Make sure the fabric is looped through the cowboy belt buckle and you have the appropriate-sized screw that came with it.
Attach the looped fabric with the screw using a flat-head screwdriver and a pair of adjustable pliers.
After screwing the tab into place over the metal loop, your cowboy belt buckle will be complete.
Make sure the cowboy belt buckle is tightened appropriately so it doesn't jiggle around or come loose on your pants while using it.