Take the window width and x by 1 1/2 to 2x (depends on much fullness you want). Fold material in half lengthwise right sides facing. Sew a seam to close. Turn inside out and come down 11/2" and sew another straight line all the way down. For a 1" rod come down from sewing line 1 1/2" and sew another straight line.
There you go! Rod pocket valance! Easy peasy!!