I've only done one sinew string, a two ply reverse twist, but it wasn't that difficult. With short fibers you tend to add new material constantly, as the legs thin out and it was pretty easy to see when you needed to add more. I used the method from the Traditional Bowyer's Bible, #1 I think, where you pinch the working end of the twist with one hand, and tightly twist the individual bundle with the other. Then you back twist it with the other bundle and advance the pinch, repeating the process with the other bundle. You only twist one bundle at a time, very tightly, and it all evens out for the string as a whole. You can feel the bundles thin out, so you know when to add more fiber. I did mine with wet sinew, hung the loop over a dowel, and clamped a weight to the end, spun it a few times to reinforce the twist and tied it off to something so it wouldn't untwist. When it dried there were lots of strands sticking out from where the new material was added. These were snipped off and the whole string sanded down to remove any bumps. The results were better than I was expecting them to be for my first try. Here's a couple pics of it on the bow. Didn't think to take any closeups of the string itself.
Julian