To determine how much fabric you need, measure the inside of the window, adding an inch to the measurement for seam allowance. Then, determine the length you want from the top down, adding another 1/2 inch for seams.
Cut out the fabric.
Fold the fabric lengthwise with right sides together.
Sew 1/2 inch straight stitch seam on all three sides, leaving 3 inches open from the end for turning.
Clip the corners.
Turn the fabric right side out and press it flat with a warm iron.
Hand stitch the opening closed.
Attach curtain clips in equal intervals and hang them on a curtain rod.