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When you've determined the height, depth and width of the cornice board, have a lumberyard cut the plywood to size. Label the top, front and side pieces with a marker.
Tip: Make the height of the cornice board 1/5 the length of the draperies.
Before assembling the cornice board, set the crown molding and corner molding in place on the front and sides of the cornice and mark their placement. This will determine the position of batting that will be attached to the cornice board before the fabric is added.
Use wood glue to attach the sides of the cornice board to the front and secure with screws. Repeat the process to add the top to the assembled front and side piece.
When the cornice board is assembled, it's ready to be padded and covered with fabric. To determine the width of the large rectangle of fabric that will cover the board, measure from one side, across the front and around to the other side. Determine the length of the fabric by measuring from the bottom front of the cornice board, up the front and over the top. Add 4" to both the length and width measurements. Seam the fabric if necessary so the pattern runs the right way.
Use spray adhesive to attach a layer or two of batting to the sides and front of the cornice board between the lines marking the edge of the decorative molding that will be applied later.
When the batting is in place, place the fabric face down and center the cornice board on it. Staple the fabric in place, beginning at the inside center edge of the front piece of the unit. Then pull the fabric taut and staple underneath the top center of the board. Secure the fabric at each side of the cornice board the same way, then continue stapling alternate sides. At the edges where the pieces of the board are joined, fold the fabric in hospital corners and staple to the inside.
Line the inside of the cornice board with a piece of muslin attached with spray adhesive.
Dress up the cornice board with crown molding at the top and trim molding around the bottom edge. Measure the width of the cornice board from corner to corner and add the thickness of the wood molding to both ends (Image 1). Use a back saw and miter box to make the angled corner cuts (Image 2). Make the first angled cut, then set the piece in place on the cornice board and measure again. Add the width of the wood molding and make the second cut. Don't make both cuts at the same time. Cut the side pieces of crown molding at the correct angle to match the front piece. Then cut the pieces of trim molding for the bottom of the cornice board, using the same method.
Prime and paint the molding as desired and let dry. After drilling pilot holes, attach the molding to the cornice board with finishing nails. Recess the nails with a nail set, fill with patching compound and touch up with paint.
Hang the finished cornice board on L-brackets and secure with screws.