I've made bowstrings from both sinew and dacron. I like my primitive bows to have sinew strings, but they are time consuming to make. I usually make a dacron string just so I can shoot the bow, then when time allows, I'll make a sinew string for it. Sinew makes a pretty good string. I've had a few failures with sinew, but I'm trying to work out the bugs with it as I am not a veteran at making sinew strings.
When I make my sinew strings, I soak the sinew strands, then chew it so it's really soft. Then I twist up the sinew into the string. You will have to add splices almost constantly. Then I stretch it, and while it's wet, I take a razor blade or a sharp flake of flint and cut off the splices that stick out of the string. Then I let it dry with a weight tied to one end so it get somewhat pre-stretched.
I like chewing the sinew because it allows the sinew to glue together, making a very even and strong string. Even still, I've had a failures with a sinew string on a particular bow, but I'm hoping to get that fixed so I can use it to hunt with.