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Measure the window from edge to edge. Add 2" to the measurement and transfer it to a 1" by 8" board. Your windows may not all be the same size, so if you're installing cornices over each window, measure each one separately.
If you plan to paint or stain the cornice, use fir or poplar for the front and sides. If you plan to cover the cornice with fabric or paper, you can use pine. Also keep in mind that the uppermost board doesn't show as its purpose is simply to keep dust off the draperies or blinds.
Cut a 1" by 8" board to the appropriate length to create the front piece, and cut a 1" by 6" board to length to create the top piece. Cut two 6" boards to create the sides.
Dry-fit the pieces of the cornice board, then mark and drill pilot holes. Assemble the cornice board with finish nails and glue and sand any rough areas.
Adding chair-rail molding or other trim to the top and bottom of the cornice will give it a more finished look. Use the cornice boards as measurement guides (Image 1), mark a width of chair-rail molding for the top and a width piece of quarter-round trim for the bottom.
Sand rough edges and cut the trim to size using a miter saw set for 45-degree outside cuts (Image 2).
Mark the back of the molding about 2" from the top on either end. With a pencil and a straight edge, connect the marks. The molding will be installed so that the top extends above the cornice. This line will help you to align the molding properly onto the cornice.
Dry-fit the longest piece of molding so that the line on the back is flush with the top of the cornice. Then drill pilot holes and attach the molding to the cornice using glue and finish nails. Use a nail set to drive the nails below the surface of the wood and fill the holes with wood putty.
Apply a coat of high-quality primer recommended for use on bare wood; allow it to dry, and apply the finish color of paint.
Determine how much of the window you want the cornice to actually cover.
Use a level to mark a straight line on the wall to demarcate the top of the cornice.
Attach the L-brackets to the top of the cornice with small wood screws; then secure the cornice to the wall using the line as a guide. Double check for level. Use drywall anchors and screws to attach the L-brackets to the wall (unless wall studs are available). Screw in the L-brackets and step away to observe your work.