Sand the Cornice Board
Using a medium-fine grade sandpaper, go over the entire cornice board to smooth any rough edges. (See first picture, above.) Pay particular attention to where the sides join the front piece; you should gently curve the joint so the wallpaper will lay smoothly on the wood.
Paint the Cornice Board
Using a color similar or complimentary to the wallpaper, paint the bottom edge of the front piece of wood and the inside of the side pieces. (See second picture, above. In this picture, we used an off-white paint.) Let the paint dry.
Wallpapering the Cornice Board
Now it's time to add wallpaper to your cornice board. Follow the manufacturer's directions for wetting the wallpaper. Starting at the left-hand side, lay the wallpaper along the sides and fronts. Smooth down with a damp sponge. Let dry. (Note: there is no reason to apply wallpaper to the top of the cornice board since it will not be seen.)
Hanging the Cornice Board
"L" brackets are used to hang the cornice board to the wall. Be careful to avoid the window casing when positioning the brackets. (See the third picture, above, of "L" bracket.) Using two brackets (three for a long window) attach brackets to wall about 1 to 2 inches above the window. (You want to cover the window covering's hardware.) Attach one brackets about 1/3 of the window length from the left and one 1/3 of the length from the right. (If using 3 brackets, attach one bracket in the middle.)Set the finished cornice board on the brackets, and attach the top of the "L" brackets to the underside of the top piece of wood.
This picture (see above, third picture) is shot looking upward from the blinds to the bottom of the cornice board. The "L" brackets are attached to the bottom side of the top piece of the cornice board. After attaching with the "L" brackets, the cornice board will be firmly attached to the wall and the top can be used as a shelf to hold plants, knickknacks, or whatever you desire.