Measure the length of the curtain rods on the door, and double that length. Measure the distance from the top of the top rod to the bottom of the bottom rod, and add four inches to this measurement. You use double the width, so the fabric will have a full look when it is gathered on the rod.
Cut a rectangle with these dimensions from a heavyweight fabric that is the same on both sides.
Iron a 3/8-inch fold on the side edges of the panel that correspond to the distance from the top rod to the bottom rod. Iron another half-inch fold on the same edges to create the hem. Sew between the folds, to hold the hem in place. Use a straight stitch and a standard stitch length.
Fold in a half-inch of fabric on the top and bottom of the panel; these are the sides that correspond to double the length of the rod. Iron the folds down. Make these folds on the same side of the fabric as the side hems. Make the casings by folding in another 1 1/2 inches on the top and bottom.
Sew the casings in place by stitching a quarter-inch from the inside folds.