Cut fabric to the desired size; ours measured the width of the windows interior with a 1 1/2-inch seam allowance. The length can be as long or as short as you desire. Line up the two fabric panels, and pin all the way around. Sew around all four sides of the panel, leaving a 3/4-inch seam allowance. Leave 8-inches open at the bottom. Trim the corners of the panels diagonally.
Turn and press the fabric. Sew the opening shut by hand. Attach the clip-on rings, and hang the curtain.