Staple the side edges of the fabric to the side edges of the cushion, stopping about 2 inches from each corner. Make a diagonal cut in the corner of the fabric toward an inside corner of the cushion, stopping about 1 inch from the cushion. Make two diagonal cuts 1/2-inch long from the first cut to finish the Y cut in the fabric. This will create a flap of fabric at the corner to cover the corner effectively. Repeat this Y cut at each corner. Pull the Y flap straight over the corner and staple it securely. Pull each edge of fabric on each side of the corner straight over the seat cushion and staple each to the backside of the cushion. Repeat the same process with each corner. This technique covers the inside corner completely.