You'll need to make a D ring, and then stitch it to the belt (fold the left end of the belt inward over the ring, and stitch it). The other end will need a smaller strap & buckle; with the buckle stitched to the inside surface of the belt on your right hip. The right end with the sreap goes through the buckle, folds inward, and the strap secured to the buckle, which is how they're adjusted for length.
A pair of belt keepers (made of the same width leather as the strap) either side of the buckle finishes the belt. If the left keeper has an oval tongue attached which rests behind the buckle when worn, so much the better (particularly for your kilt).