To make a window cornice, choose a suitable wood type, molding style and finish that suit your home; gather the necessary materials, such as poplar and aspen; build the basic box; and fit the trim. Use wood filler to fill nail holes and joint gaps, and mount the cornice carefully.
Depending on your chosen design, the top of the cornice may overlap the sides or feature a routed edge. It can also be nailed to the interior of the box. Combine the right top style and board size to avoid cutting the top to width.
Poplar and aspen make good box materials if you intend to paint the cornice, because these materials are stable and affordable. For paintable trim, choose pine and basswood molding. Maple, mahogany and oak are excellent choices for a stained or natural-looking wood cornice.
Add 1 inch to measurements of the outer depth and width of the curtains and curtain rods. Build a lidded box with three sides, add your chosen trim, and finish the cornice. Get rid of wood fibers and make the surface smoother by sanding extra wood filler. Use multiple light coats of spray paint if the cornice features intricate details. To mount the cornice to the wall, screw it to a 1 by 2 that is attached to the wall above the opening.