On all my belts there is a small buckle riveted to the inside surface of the belt and a small strap stiched to the end of the belt. The small strap passes through the buckle and provides the adjustment. Use the best grade of buckle you can get in order to minimise any rough edges that might abrade the kilt. Keep in mind that a kilt belt on a properly fitted kilt is primarily for show and shouldn't be so loose as to shift around a lot or so tight to "dig in" uncomfortably at the hips and possibly wear or tear your kilt.
Jamie
Quondo Omni Flunkus Moritati