- Measure the width of the window to the outside edges of the trim for the front board. Add 2 to 4 inches to the width, allowing enough space to attach the cornice to the wall.
- Measure the depth of the end pieces from the wall outward, allowing for drapery or shade clearance.
- Cut plywood pieces to your dimensions, making one front board and two side pieces. The height of the pieces should be at least 8 inches and no more than 18 inches.
- Cut a top, or dust board, for the cornice box using your length and depth dimensions.
- Place a side piece along each inside end of the dust board. Attach the pieces using wood glue, keeping the edges flush. Secure the pieces with nails.
- Attach the front board using wood glue, making sure all edges are even. Secure with nails. You should now have a box with an open back and bottom.
Upholstering the Cornice
- Cut thin foam to the size of the cornice, adding a 1-1/4-inch hem allowance all around. Glue the foam to the front and sides of the cornice box. Fold and glue the 1-1/4-inch hem allowance to the top and back of the box.
- Cut the fabric for the cornice to the dimensions of the box, adding a 2-inch hem allowance all around.
- Lay the fabric on the cornice, centering any designs or patterns. Fold a 3/4-inch hem allowance to the back of the board. Pin the fabric in position.
- Staple the fabric beginning along the top edge of the box. Staple from the center out. Make folds to miter the corners as if wrapping a present. Secure with staples. Repeat the procedure along the bottom of the box.
- Measure a length of decorative piping to go along the bottom and sides of the cornice. Hand stitch the piping along the inside lower edge of the cornice.
- Cut lining fabric to the measurement of the inside of the cornice box, adding a 5/8-inch seam allowance all around.
- Place the lining fabric in position, folding under the 5/8-inch seam allowance. Staple in place being sure to encase all raw edges.