There are different ways to conceal or hide the drapery hardware that suspends your curtains. You may choose to install the curtains high enough so the top edge of the material covers the tracks, but this is not your only option. A cornice box fits over the track to hide the hardware even when the curtains are open. Material on the exterior of the box adds a decorative touch. This is a home improvement project that most you can complete on their own. If you can use a hammer and nails, you can likely build a cornice or valence box. Cornices also serve a practical purpose in closing off drafts from around the top of the window.
To make a wood cornice, start by measuring the width of the track you want to cover. Add 1-1/2 inches to this length for the total length of the cornice’s top and front boards. This allows for enough room to install the cornice. The board that will go on top should measure at least 5-1/2 inches long, which will be the depth of the cornice. This lets the cornice fit completely over the track. Make the height of the cornice at least 12 inches to allow for 8 inches above the top of the track and 4 inches below. This height is the height of the front board. Subtract 3/4-inch to find the height of both side boards. The sides need to be as wide as the board that goes on top of the cornice.
After measuring, cut 1-inch boards to your measurements. You might want to lightly pencil the location of each piece: ie front, top, sides. This helps you to keep things straight as you work.
Nail the two sides to either end of the top board. Use a protractor or square to ensure that the top and side boards are at perfect 90 degree angles to each other. Affix the front board to cover the edges of the top and side boards and check the angles. Use craft glue to glue batting over the front and side boards of the cornice and wait until it dries completely. Trim off any excess batting.
Affix fabric to the entire exterior of the cornice by wrapping it around and stapling it through tack strip on the back side. Attach the cornice to the wall above the curtain track with wall angle brackets placed every 36 to 40 inches along the length of the cornice and at either end.
Source: Sunset: Curtains, Draperies, and Shades; Sunset Books; 2002