Applying a wallpaper border around a window or door creates a distinctive, decorative frame. But you'll need to cut perfect miter joints where the border strips meet at the corners for professional results. Here's how: Start by positioning the border so its most prominent design element — a large flower or diamond, for example — won't fall in the middle of the corner joint. Apply the strips, letting them overlap at each corner. Then hold a 6- or 8-inch drywall knife or other metal straightedge diagonally across the joint and cut through both layers with an artist's matte knife or utility knife. Be sure to use a new, sharp blade. Carefully remove the cutoff waste piece from the horizontal top strip. Temporarily peel back the horizontal mitered section that will remain and remove the upper, vertical waste piece beneath it. Then put the section back and press the seam flat with a seam roller.