I recently posted photos of my new curtain in my also new "Reading Room".

I found a wonderful "how-to" from Living Solutions The Blog: Ideas and Inspiration

I altered her tutorial just a bit and made my own box pleat curtain.  She does a great job describing how to measure how much fabric you will need with the addition of the pleats, hems, etc.

For my curtain I only wanted 1 pleat.  For each pleat you need to add 8 inches of fabric.  So, this is how I did my math:  my window is 46" wide.  I wanted my valance to be 47" wide (just wider than the window).  So my fabric needed to be 47" plus I added 8" for 1 pleat, 1/2" for 1/4" hem on each side, 7" for the 3 1/2"'s on each side of the board to wrap around and an additional 1" to wrap around to the back of the board to staple it.  Therefore my fabric was 63 1/2" wide.  

I wanted my valance to be 16" high.  I measured 16" plus 3 1/2" for the width of the board it would attach to, plus 1" for hemming allowance around the top and bottom.  So my fabric was 20 1/2" high.

I used a Target curtain panel for my fabric.  I happened to be walking through Target and found a curtain panel for $20 in the perfect color.  For the amount of fabric you get, it is quite a deal!

Measuring before cutting my fabric (63 1/2" X 20 1/2")
Finding the center of my fabric and pinning my pleat.
Pleat pinned - each fold is 2"
I factored a total of 8" for my 1 pleat

Before sewing, iron your pleat(s)!
After sewing my pleat into place - I sewed down the middle on both sides about 6 inches
View of the sewn pleat from the back
I then placed my curtain onto my lining fabric and cut my lining fabric to the same size.  I then pinned the right sides together of my lining fabric and my curtain fabric
I then sewed around the top and 3/4" of the way around the sides of the fabric

I did not sew around the bottom or 1/4 of the sides.  This way I could hem them separately.

The curtain has been turned right side out and ironed
In this picture you can see that I still need to hem my lining fabric and then sew together the sides
I then hemmed the lining and outer fabric.  For the outer fabric I sewed a blind hem after serging it.
Blind hem completed on outer fabric
Blind hem completed on outer fabric (you can't see the stitching!)

I serged the lining fabric and then sewed down the sides of the valance to finish the sides.
Sewn down the sides of the valance to attach the the lining and outer fabric

View from the back
Views from the front

Views of front with back folded over 

After the valance is sewn and ironed, you need to attach it to your board.  I used a staple gun to attach it to my wood 2x4 board.  I did it exactly as is shown on the Living Solutions Blog:

She explains in detail how to wrap your corners.

My finished valance!

For more photos of the completed valance in place visit the original post:  Box Pleat Valance - DIY