Measure the width of the window frame or alcove.
Adjust the length of a tension rod to 1/2 inch longer than the width measurement. A wide-diameter shower-curtain rod makes a good choice for this situation; it'll be more stable than a standard drapery rod.
Adjust the width of the casing at the top of the drapes, if necessary, to fit the diameter of the rod. Remove the seam at the bottom of the existing casing with a seam ripper. Do not unfold the fold in the fabric at the bottom of the casing. Measure the diameter of the end of the tension rod. Add 1 inch to this measurement. Make a new casing by folding down the top edge to match this measurement.
Set a sewing machine to sew a straight stitch with a standard stitch length. Sew 1/4 inch from the bottom fold to hold the casing in place.
Thread the tension rod through the casing.
Compress the tension rod by pressing the ends together until the rod is 1 inch shorter than the width of the window. insert the rod in the top of the window frame or alcove. Release the rod and allow it to extend along the width of the opening.