The windows I made these for measured at 57" each. For valances you double your width so that makes it 114" for each window, 228" inches total of fabric which comes out to be 6.33 yards of fabric, which I rounded to 6.5 yards of fabric. It's better to have too much than not enough. Plus I cut off about 4" to practice on and get the tension of the thread right on my sewing machine.
So now you have 6.5 yards of fabric, start out by cutting that in half so you have 3.25 yards of fabric. I used pinking shears on each end. So I'll start with the first one, having both ends cut by pinking shears. This will be the side of your valence.
Fold the fabric over so you can make a clean edge. I just eye balled it all the way through, but you can measure. Be sure to iron and if you want to pin it you can, but I didn't. Then sew the edges.
Next fold the fabric in half. I decided I wanted my valances double thick since the material was a little thin. So fold it with the pinking sheared sides on the inside and sew the top as shown below.
So now you basically have a long tube with the sides finished. Next it's time so sew the opening that the window rod will go through. Mine is about 2.75" and you need a little extra room so I decided to make the opening about 3", with 3" sticking up the top. So the top edge that you just sewed is folded over making it 6". I decided to do it this way because this makes it a better length for the window. So I measured 6" and pinned it every few inches.
Sew this about 1/4" in, I just eye balled it again and used the sewing machine foot thing as my guide.
Next measure 3" from the stitching you just did and mark it with a marking pencil. I decided to mark a few inches and the start sewing and mark it as I went.
Once you have finished this, your curtain is done. Be sure to iron it since it probably got wrinkled from handling it. Then do the exact same thing with the other one and hang them. Here is what they look like finished.
If you have any questions be sure to ask me. I have used this same technique to make valances for my living room, dining room, bedroom and bathroom. For the length of the valances it's pretty much up to you on how long you want them. I don't know exactly how long they are, I just eye balled it. For the width, I've found that twice the width gives you about the right amount and the right look.