Measure the width of the window to get a rough idea of how many place mats you need for the window.
Fold each mat down one corner and sew them up in the shape of a triangle, leaving some space so that the curtain rod can go through them easily, much like normal drapes. Do not connect the place mats to each other, but simply overlap them against each other, placing one after the other on the rod in a triangular fashion. You can even choose to connect them using some kind of strong glue.
Adjust the place mats until you get the desired valance effect. Place mats will give the window an ascot valance look.
Embellish the ends of the mats by adding some frilly lace or a tassel to either side of the window using double-sided carpet tape.