Make sure the backing is several inches wider all around than the front. Fold the point of the backing at the corner toward the front until the point of it touches the point of the top corner. then fold the four sides of the backing toward the front until they touch the edge. You should then be able to fold the back to the front neatly all the way around and whip stitch or machine sew it down . There should be no raw edges showing and the corner pieces should butt up against each other neatly. Think of the way that you would wrap the lid of a gift box in order to make tidy corners. It works the same way only fabric is much more forgiving.