I put the straps on the apron (has fringe) and underapron (no fringe) with carpet thread.
I attached the buckles to tabs of fabric and sewed those onto the back. The one on the left is just behind the “buttonhole.” The underapron strap goes through the buttonhole and uses that buckle. The apron strap wraps around the outside and uses the buckle on the right. The loops in the middle are NOT for a belt, but for a sporran, a traditional pouch that is worn in the front. You can wear a belt with a kilt, but it should go on top of the kilt and not be attached to it.
It really looks like a kilt now. Almost done, but next is the dreaded lining!