Position the first curtain stem piece and mark its screw holes on the wall with a pencil. The curtain stem piece should be as close to the actual window as possible, with no more than a 6-inch space between the window and the piece. This ensures the curtain rod is anchored into the window's supporting wood.
Position the other curtain stem piece and verify it is level with the other side by setting a long level on the top of the piece. Once the level's bubbles are centered, mark the screw holes.
Drill pilot holes where the screw holes are marked with a drill bit that is slightly smaller than the actual screw.
Position the stem pieces and screw the pieces into place.
Feed one of the rod pieces through the curtain's top and set the rod onto the stem piece. Repeat this with the valance, placing the valance rod in front of the curtain rod.