This is a very popular style valance, that works with any size window. I like to hang mine about 4" above the window and make it aproximately 4" wider than the window (including frame). If you have curtains, you will want to adjust the width of the valance to accomodate them. The finished length can be anywhere from 10-16", depending on the height of the window. A good rule of thumb is to have your valance aprox 1/5th the total length.
These instructions are for a mock roman valance with a finished length of 12".
1. Cut fabric the finished width (remember to include the returns on the mounting board!) + 4" for side hems x 32 1/2" long. Cut lining the finished width x 28 1/2".
2. Press hem on fabric 2" twice. Place lining on wrong side of fabric, lining up the top end and tucking the bottom under the folded hem. Blind stitch, either on your machine or by hand.
3. Serge the top edges together and the side edges together. Press the side edges in 2" on each side. Blind stitch hem. Sew velcro to the top on the right side.
4. Mark your stithing lines with pins at each end of the blind. Starting 4 1/2" from the bottom, place 1st pin. Then place pins every 2 1/2".
Start with the lowest pleat, lining up pins.
Flip the valance to the wrong side. There will be 7" between the fold and the bottom edge. Pin and stitch all the way across the valance, 2 1/2" from the fold.
Press the first pleat down and fold the 2nd pleat by matching pins again.
Flip the valance to the wrong side again. There will be 5" between the fold and the first stitching line. Pin and stitch 2 1/2" from fold.
Press the 2nd pleat down and fold the 3rd pleat by matching pins. Stitch the same as you did the 2nd pleat. Once all 3 pleats are sewn it should look like this:
I like to use either 1x3 or 1x4 to mount the valance to. I staple the male side of the velcro to the sides and front of the board. If mounting above the window, simply attach L-brackets to the underside of the board, position above window, and screw in place. Attach valance by turning the velcro to the wrong side, so it will not be visible.