Measue the width of the sidelight window from the inside of one side of the frame to the inside of the other. Multiply this measurement by three. Measure the height of the window from the inside of the top of the frame to the inside of the bottom of the frame. Add 3 inches to this measurement.
Cut a rectangle of sheer fabric with these measurements.
Fold down 3/8 inch on both the side edges of the fabric. Iron these folds. Fold down and iron another 1/2 inch to make a hem.
Set a sewing machine to sew a straight stitch with a standard stitch length. Sew 1/8 inch from the inside folds to hold the hems in place.
Fold down 1/2 inch on the top and bottom of the sheer and iron these folds. Fold down and iron another inch to make a casing at the top and bottom of the curtain.
Sew the casings in place by stitching 1/4 inch from the inside fold.
Unscrew two tension rods until they're 1/2 inch longer than the width of the sidelight window. Thread one rod through the top casing. Press the ends of the rod toward each other to compress the rod. Insert it into the window frame as close to the top as possible. Thread the other rod through the bottom casing and place it as close to the bottom of the window as possible. Adjust the gathers of the curtain evenly along the rods.