use a compass to scribe the curve onto a scrap piece. transfer this to the Luan. shift the scrap piece over to draw the outside curve at the correct distance top and bottom. also draw the re-enforcing strips. glue them to the curved panel and let dry. now glue the front face to the side. use brads to hold it in place. check each window for correct curve. the valance should fit against the plastic window frame. the bottom of the valance will just hit the bottom of the plastic. good luck.