To miter each corner, lap one border strip over the other. Align the edge of a 90-degree right triangle with the raw edge of the top strip so the triangle’s long edge intersects the border seam in the corner. Draw along the triangle edge from the seam out to the raw edge. Place the bottom border strip on top and repeat the marking process. With right sides together, match marked seam lines and pin. Beginning at the inside corner, sew together the strips, stitching exactly on the marked lines. Check the right side to see that the corner lies flat. Trim excess fabric, leaving 1/4" seam allowance. Press seam open.