I make cordage from back-strap sinew.
It take a bit of time but is worth the effort.
Get your sinew damp, not wet just damp, and cord it up.
You can dampen it by rubbing thin hide glue on it and it will hold togeather much better.
Bit sticky to work with. Keep a wet towel close buy to remove the glue from your fingers.
Once you get a foot or so corded stretch it with your hands and hold it over a fan for a minute or two. That way it will dry straight. You can roll it up later after it is completely dry.
David