Measure the length of the bar at the top of the blind and the distance from the wall to the front side of the bar.
Purchase two 3/4-inch curtain rods that are 1 inch longer than the length of the top of the blind. The reach of the rods should be 1 inch more than the distance from the wall to the front of the blind. The reach on a curtain rod is the distance from the wall to the front side of the curtain rod.
Hang the curtain rods over the strip of hardware at the top of the blinds. Place the brackets that come with the rods against the wall and mark the position of the holes with a pencil. Drill a hole for the screw anchors at each mark, and pound the anchors in with a hammer. Place the brackets over the anchors, and use a screwdriver to screw the screws through the holes in the brackets and into the anchors. Hang the top rod 1 inch above the top of the blind and the bottom rod 1 inch blow the bottom of the bar at the top of the blind. The ends of the rods should extend 1/2 inch beyond the blinds on each side, and the front of the rod should be extend 1 inch in front of the top bar.
Measure the total length of the rods, starting at the wall on one side and measuring out and around the front and back to the wall on the other side. Measure the distance from the top of the top rod to the bottom of the bottom rod -- the height of the valance.
Cut a rectangle of fabric 1 1/2 inch longer than the length you measured and 3 1/2 inches wider that the height of the valance.
Set the machine to sew a medium zigzag stitch. Finish the raw edges of the fabric by sewing a zigzag stitch along all four sides.
Place the fabric right side down on an ironing board and iron a 1/2-inch hem on both side edges.
Set the machine to sew a straight stitch with a standard stitch length and sew 3/8 inch from the folds to hold the hems in place.
Make a casing for the curtain rods by ironing a 1 1/2-inch fold on both the top and bottom sides of the fabric. Sew the casing in place by stitching 1 inch from the folds.
Take down the curtain rods and thread them through the casings. Rehang the rods. Pull the sides of the fabric to the wall on each side.