Measure down from your curtain rod to decide the length you would like the valence. Generally, 6 to 8 inches would be an average length. The larger the window, the longer the valence. Therefore, if you have a very small window, the valence may only be 5-inches long. Now, measure your window across. For fabric purchasing, double your window in width and add an extra 5-inches to the length. The valance should be all one piece. For larger windows you may have to get two lengths of fabric and sew a seam in the center to reach the needed width.
Cut your fabric evenly to the same length all the way across. Now, you should have one long strip of fabric, considering you may have sewn two fabric lengths together for that large window. Start by pressing ¼-inch under, along the outside edges. Then fold the fabric over another ¼-inch and press. Sew close to the inside fold. This will make nice finished edges to your valence window treatment.
Fold under and press ¼-inch along one side of the long length. Fold, in the same direction another 4-inches and press. Measure down 2-inches from the folded edge and stitch across the length of the valence. Stitch across again, this time close to the folded inside edge. This will form the valence header (top ruffle) and the tube that will hold the curtain rod. These measurements should be adjusted according to your window size, curtain rod width and your taste. There are no exact rules about these measurements.
Measure from the top of the valence to decide the proper length of your window treatment. Allow 1 ¼-inch for the hem and cut any additional fabric off, straight across. If you have a rotary cutter and mat, use them for an easy way to cut straight lines. Otherwise, measure and mark the fabric and then cut with your scissors.
Fold the raw edge of the fabric under ¼-inch. Measure another inch, fold under again and then press down. This forms the bottom hem to the valence window treatment. Sew across, lengthwise, close to the inside folded edge. Press the entire valence before hanging it on the curtain rod.