I'll try to help.
First, you don't "need" a board/jig--it just makes life easier, especially if you make a lot of strings.
Going by a bow's "AMO" length is hit or miss. AMO says 3" shorter, longbow or recurve, but there's a lot of bows that don't meet AMO specs. I've made strings that ranged from 1.5" shorter to 6" shorter than the AMO bow length. If possible, know what length string you need to start.
Take the actual string length, then add 16" or so. More won't hurt--you can drop strands like Joe does in the DVD (Doin' the Twist) if needed.
The problem you are having (string looks like two ropes wound around each other) is caused by a lack of back-twist. After you complete the first loop, untwist the bundles until they are straight. Count the number of twists it takes to get the bundles straight, then put the same number of twists back in, but in the opposite direction. This should get you close--a little experimenting (take notes) will get you right on, if you do it the same every time.
The reason you do this is when you build the second loop, you'll be putting twists in the bundles but you can't get these out--the reverse twist will work out as you build the second loop. When you get done, your bundles should be hanging straight (if you did it right), and will twist up like one string.
If you learn to do this properly, a 2 bundle will come out just as round as a 3 bundle.
I purposely make my strings about 2" too short. I have a stretching jig that will pre-stretch the strings and get that much slack out of the twist and the material. It's also really handy for serving strings and tying on silencers.
Everyone is a little different, and different materials will act a bit differently.
A usable string is very easy to make. A really good string takes a little practice, but isn't much harder.
Chad