Measure and mark the width of your window and add one inch for a hem on each side. Decide how many pleats you will add and measure for each pleat, adding this number to the number for the original width plus side hems.
Measure and mark for how long you want the valance to hang and add at least two inches for a hem. Remember the valance is a window topper that usually hangs a third of the way down the window to allow light to pass through.
Cut the fabric according to your measurements and pin the pleats in place. Carefully iron the pleats and seams in place.
Sew the top hem first and remove pins as you go. Sew the side hems and finish with the bottom hem. Carefully follow the pleat lines to ensure they will sit forward as you sew along.
Pin a length of trim in place and either sew it or use a glue gun and secure it with hot glue. Use the curtain clips and hang the valance onto the curtain rod. Once the curtain rod is in place adjust the valance and fluff the pleats into place.