Push the top strap through the rear of the top hole and back out through the bottom hole. Then push the bottom strap through the rear of the bottom hole and out through the top. Pull tight.
I would guess both go up through the rearward hole and both come over the middle and through the front hole. The rear strap stays relatively straight, the front loops back in a C shape. But I've always had one strap permanently attached to the buckle so not entirely sure if that's secure