How to Make Cornices for Windows Out of Foam Core Board

Measure the length of the curtain rod on the window you want to install the cornice. Add 4 inches to the length to get the length of the cornice. If the curtain rod is installed inside the window frame, measure the width of the window frame from side to side, and add 4 inches to that measurement.

Cut out a piece of a 15-by-1-inch foam core board that is the length of the measurement from Step 1 to create the front of the cornice. Use a utility knife to make the cut. Depending on the foam you are working with and the sharpness of the knife blade, it may be easiest to run the knife down the length of the board three or four times, cutting deeper as you go, rather than cutting it all in one pass. To keep the cuts straight, use a metal ruler or yardstick, and run the knife against it as you cut.

Cut a second piece of foam core board measuring 6 by 1 inches with a length equal to the measurement from Step 1 to create the top of the cornice.

Cut two more pieces of foam core board to create the sides of the cornice. Make the pieces 6 by 14 by 1 inch each.

Apply a line of hot glue down the length of the top piece on one of the 1-inch sides. Lay the piece flat on your work surface. Turn the front piece perpendicular to the top piece, with the 1-inch side down. Press it against the glued edge of the top piece. The edges and corners must be flush. Hold it in place for three to five minutes while the glue dries.

Apply hot glue to the 1-inch sides of the side pieces. Press the side pieces into the corner of where the front and top pieces meet. Hold the pieces in place for three to five minutes to allow the glue to dry.