Measure the distance from one end of the curtain rod to the other to calculate the width of the box. Add 1 inch to the measurement and note it. Measure the thickness of the curtains, plus 1 inch, to determine the depth of the box.
Cut the 1-by-8-inch lumber for the front of the curtain box to noted measurement using a miter saw, forming 45-degree bevels on its ends. Also cut one end of each short, or depth, piece at 45 degrees. Reset the saw to 90 degrees and cut the other end of each short board to the required length, ensuring they are identical.
Apply wood glue to the mitered edges of the front board. Join the mitered end of each side board to the glued ends of the front, and press the boards lightly to form a secure bond. Let the glue dry for the time specified on the label directions. For added stability, tack three 1-inch brads through each side piece that penetrate the edge of the front board.
Measure the width and length of the exposed surface along the top of the forming curtain box to determine the dimensions for the top board.
Measure and cut a length of 1-by-8-inch lumber to the required size for the top of the curtain box. Center and lower it over the cavity, checking to see if it fits flush against the edges and sides of the structure. If required, trim excess wood along its length and width. Set the board aside.
Apply wood glue along the exposed upper edges of the front and sides of the curtain box to which you will attach the top board. Center and lower the board over glue, pressing it gently to secure the bond. Let the glue dry for the time specified on label directions.
Inspect the curtain box for inconsistencies in its design or assembly and correct them to give the structure a smooth finish. Insert wood filler into gaps between mitered joints and sand the excess until it is flush with the surrounding wood.
Apply a coat of primer over the wood curtain box and let it dry. Sand the surfaces lightly before painting them.