I leave my border ends just slightly longer than they need to be then from the back using a straight edge draw a line from the stitching line on the quilt to the spot where the border pieces intersect. Fold the quilt, right sides together, so that the border pieces line up straight (it will look like a giant triangle) then stitch on the line drawn. Turn and make sure they lay flat then trim the seam to a quarter inch. So far it has worked well and I haven't had any wonky mitered corners.