To make a simple valence for a standard single window:
You need one yard of fabric
Thread (to match or not is up to you)
Step 1 - Iron your fabric
Step 2 - Fold the fabric in half - inside out (Hold it up to your window to see which way you'd like to fold it. Either way works. It will just be a different length.)
Step 3 - Sew the long end shut using a consistent seam allowance (usually 1/2 inch works well)
Step 4 - Sew the short ends shut starting at the ends with the sewn seam. Stop about 3 or 4 inches from the folded edge. (leave enough for your curtain rod to slide through there)
Step 5 - Turn the curtain right side out
Step 6 - Iron the curtain, paying special attention to all the edges.
Step 7 - Tuck the 3 to 4 inch gap inside and iron it into place so that all the edges are even from top to bottom.
Step 8 - Hang your new valence.
This type of valence looks best on a pocket curtain rod like the one in this picture. They cost about $8. (Click it to go to Lowes site)
If you feel like the valence needs a little something extra, you can loop a ribbon around it in two or three places to pull it upward and give the bottom a scalloped appearance. (This one is hung on a regular curtain rod. You can see how it hangs differently. It was also probably more than one yard, which is why it's a little longer.)