Determine your desired dimensions for the valance box. It should be tall enough to hide any curtain rod, and deep enough to allow blinds or drapes to operate properly underneath it. The width of the box must be at least 4 inches wider than the window, including the window trim, so that the box can be mounted to the wall. Measure the window, including trim, and add 4 inches (2 inches for each side).
Create a pattern for each separate piece of your valance box (top, front, and two sides) by sketching each piece's measurements on your poster stock. The side pieces should have the same measurements. The top and front pieces should be the same length, and the top piece should measure the same depth as the side pieces.
Cut out each pattern piece using your utility knife.
Lay each pattern piece onto a piece of wood. Trace around each piece with a pencil, then use your saw to carefully cut the pieces out of the wood.
Lay the front piece of the valance box flat on a stable surface. Stand one of the side pieces on its edge, placing it on top of the short end of the front piece. Make sure the edges of the boards are aligned, then position an L-bracket where the pieces meet. If desired, use the electric drill and 1/4-inch drill bit to drill a shallow pilot hole through a hole on each side of the bracket and into the wood. Screw a wood screw through a hole in each side of the L bracket. Repeat this step for the opposite side.
Lay your top piece flat on the work surface. Set the three-sided piece on top of it with the short sides facing toward you. Make sure the long edges of the boards meet at the rear and the side edges meet the ends of the top board. Use screws to attach the remaining L-brackets to the wood, as in Step 5. Use one bracket on each side and two to connect the top and front boards.
Sand your valance box and finish it with stain, paint or fabric.
Securely mount your valance box to the wall near the top of the window frame with L-brackets and screws. The top of each L-bracket should extend outward from the wall, parallel to the floor, with the other leg of each bracket sitting flat against the wall. Attach the brackets to the wall with screws, then sit the valance box on top of the brackets. From underneath, screw a wood screw through one of the bracket holes and into the top board of the box. Repeat for the other side.