After you finish your first loop, there will be several twists in the bundles. Count how many twists it takes to get the bundles hanging straight, then put that same number back in, but in the opposite direction--that is back twist. After you finish your second loop, your bundles should be straight again (or very close). That will allow the bundles to go together as one nice, round, smooth string when you twist it up.
My stretching jig is made from square tubing. Basically two "L" pieces, one fitting inside the other for length adjustment. Drill a hole or two, weld a nut over the hole, then add a bolt to lock it in place at the desired length. A screw hook on each end is used to hang the loops over, then tighten down on those to put tension on. Works great, and you won't need a different bow for every length string.
I've burnished strings with leather myself, but the manufacturer's strongly recommend against that--too much heat can damage the material. The jig shortens the break-in time greatly.
The plans for it are on the DVD "Doin' the Twist", but if you need more info. just shoot me an e-mail and I'll send some pics of mine.
Billy, 12 strands of most any string material is fine--actualy a bit much with some materials. I'm assuming you are talking about dacron--as long as you aren't pulling over 50#, 12 strands of dacron is fine (it's going to stretch more with a heavier draw weight).
Chad