Measure the width of your door from the outside of the door molding. Add 8 inches. Cut a 1-by-4-inch board to this length with a table saw.
Screw L-brackets into the 1-by-4-inch board. Screw one bracket at each end and evenly space additional brackets every 24 inches. Position the board with the 4-inch-side down and the length horizontal to you. Place the L-bracket so that one leg of the bracket is flush against the board and the upright portion is flush with the side edge of the board. Screw all of your brackets into place.
Lift the valance board above the door so that the L-brackets are facing down and the back leg is against the wall. Center and level the board. Mark your bracket holes. Insert wall anchors into the wall and screw through the brackets into the wall anchors to secure the board to the wall.
Measure the length of the cut board. Measure along the side of the cut board and multiply the amount by 2. Add 1 inch. This is your total length measurement for a flat panel valance. Measure from the top edge of your board to the top of your door. Add 4 inches. This is the height of your valance. Cut your fabric to these measurements.
Fold your fabric in half along its length with the face sides together. Sew a 1/2-inch seam along each side. Turn your valance right side out and iron. Turn the top edge under 1/2 inch and sew a seam.
Position the valance fabric centered on the valance board with the top edge folded over the top front edge 2 inches. Check your height to make sure your door can swing easily past the valance. Staple the valance to the top of the valance board. Fold your corners neatly and continue stapling around both corners. All of your staples should be on the top side of the board and out of sight.