OK, now I can help you here....
How much BOC did you buy? Enough to cover the width you want twice? That's how I did it. Take the two widths 54" and sew then together the long way to make 108" tall piece that is wide enough to cover the span of your frame.
SO, if you are going 12' wide (4 yards) you'd need 8 yards of fabric. Cut it in half the short way and then sew those two pieces together the long way to end up with 108" X 144".
Actually you'll end up something about an inch short as you need to make the seam. You shjould only about 1/2" from each piece to make the seam. DOULBE sew it to give a little ectra strength. You can use regular old cotton thread, but there is heavier duty avavilable. Because it is, well, BOC, you will not see the back of the seam (the two pieces actually sewn together) from the front.
You'll end up with one long seam accross the screen, but it will be virtually invisible, except in REALLY bright scenes, where your eyes are focused on that spot and there's no other action. I've only noticed it once or twice in scenes with diaglog that stay on an actor's face for more than few seconds.
It's really not that hard. Just pin everything first and it take two to sew...one to sew and one to help manage the fabric as it is being sewn. Don't worry, you'll get equally stuck with pins!