Measure the width of your window opening or curtain rods and make a note. Then decide how long your valance will be. Starting at the top of the window opening or curtain rod, the valance should be between about 12 and 18 inches deep.You can make it as long as you like so long as it fits your window.hen decide how long your valance will be. Starting at the top of the window opening or curtain rod, the valance should be between about 12 and 18 inches deep. You can make it as long as you like so long as it fits your window.