Measure the length of your curtain rods. Add six inches. This measurement will be the length of your cornice box. Then, starting at the end of the left side of the curtain rod, measure out 3 inches along the wall. Mark this spot with your pencil. Repeat on the opposite side. These marks represent where your cornice box will attach to the wall.
Measure from about 2 inches above the curtain rod to the top of the window casing. This measurement will be the height of your cornice box.
Cut one piece of plywood to the length and the height determined in the previous 2 steps. Cut two more pieces of plywood. Each of these two pieces should be 8 inches (this is the depth of your curtain rod box) by the measurement determined in the previous step (the height of the box). These are the box ends.
Unroll your fabric face down on the work surface. Place one of your plywood boards on the fabric. Cut a piece of fabric about 3 inches longer and wider than the board. Repeat with the remaining two boards.
Unroll the batting on your work surface. Cover the front of one of your plywood boards with spray adhesive. Place the board adhesive-side down on the batting. Wrap the batting around the board, as you would wrap a gift. Secure the batting to the back of the board by stapling around the perimeter. Repeat with the other boards. Leave the right and left edges of the boards free of batting.
Place your fabric pieces cut in Step 3 face down on the work surface. Apply fabric to the boards using the same methods described in Step 4.
Stand the long board up on its bottom edge. Apply a thin layer of wood glue to the right edge of the board. Line up the right edge with the edge of one of the small boards. Secure the boards together with a corner clamp. Allow the glue to dry completely. Repeat on the left side.
Screw a right-angle bracket just inside of the pencil mark you made on the right side, above the window in Step 1. Screw the second bracket just inside of the pencil mark on the left side. Hoist your cornice box to the brackets. Line up the open ends of the brackets with the sides of the cornice box. Screw them in place to hang the box above your window.