Measure the length and width of your window. Measure out a piece of fabric that is about three times the width of your window and quarter as long as your window. Cut out the fabric.
Fold over the side hems by half-inch on the underside of the fabric and iron them flat. Fold the hem over again by half-inch and iron it flat again. Pin the hem in place.
Sew the side hems with a one-eighth-inch seam and a straight stitch on normal tension.
Fold the bottom hem by 1 inch and iron it flat toward the underside of the fabric. Fold the hem up another 3 to 4 inches and iron it flat. Pin in place and sew with a straight stitch on normal tension. Use a one-eighth-inch seam.
Fold the top hem over by half-inch and iron it flat. Turn the hem over again by 2 to 3 inches and iron it flat. Pin in place and sew the hem in place with a one-eighth-inch seam.