Slabs of styrene can be found at most craft stores and come in various lengths,
widths, and thicknesses. You’ll also need glue, toothpicks, putty knife, fabric, foam, welt, or other decorative touches. Lay the styrene slab on a table and measure the length of your window. Transfer this measurement to the styrene, now measure how far you want it to come out from the wall. This should be no more than a few inches in front of the curtain rod.
This is your top piece, which goes over the rod and the length of the window. Now measure how far down you want the design to go and transfer this measurement to the styrene, along with the length of the window once again. This will be your front board which sits in front of the curtain rod and attaches to the piece above the curtain rod. The styrene is easier to cut if you get a serrated knife and cut through an old candle first. The wax makes the knife pass through the styrene with ease.
Decide at this time if you want the cornice board to be straight across the front bottom or if you want to cut in design. To make a wavy edge, draw the pattern on the styrene with a magic marker, then wax your knife and follow the pattern. After cutting the pattern for the front piece, you might want to extend the pattern to the bottom of the side pieces as well. The measurement for the side pieces will be the length between the wall and how far out you want the cornice to go and the measurement from the top of the front board to the bottom of the front board. Cut your side pieces and cut in your bottom design. You should now have a top board, a front board with design cut at the bottom, and 2 side pieces with designs cut in at the bottoms.
To assemble the cornice board, dip toothpicks in white glue and insert them, about halfway, into the front piece of the cornice, on the top edge. After you’ve inserted the toothpicks all the way across, about every 4 inches, lay the cornice top piece atop the toothpicks, then press the two cornice pieces together at a 90-degree angle. Now slide the side pieces into place and dip toothpicks in glue before inserting the toothpicks from the front piece into the side piece, and from the top cornice piece into the side pieces.
Now decide how you want the front board to look. You can use a putty knife to create a crevice across the front, giving the front board a sectioned look. Go down two-thirds of the way from the top on the front cornice piece, then starting at the end, push the putty knife down, creating a crease all the way across. Now lay foam or cotton across the entire front crevice piece, letting it hang over the top and bottom temporarily. Use the putty knife now to slide the foam and fabric into the crease. The foam will give it a padded look when finished. Let the foam or cotton hang over the top and bottom of the cornice board until the creasing is done, then trim.
Wrap the fabric around the edges of the styrene, on the front bottom and sides, and around the back of the top cornice piece. After all the fabric is tacked down, you can also add welt, cording or other decorative touches across the crease of the cornice. To hang, simply screw 2 brackets in the wall, one on each side of the curtain rod, and place the cornice board on the brackets. For an exceptionally long window, add a middle bracket before hanging.Wrap the fabric