To get the paracord flat, you have to cut off what you need then start working the core out of the outer sheath. I believe needlenose pliers is the best to grasp the core. You have to sort of work the core out, you can't just grab the core, grab the sheath and zip it out. Once the core is out, wet it then hang a weight(s) on the ends to get the stretch out.
A great guy name of Brian Vanspeybrock uses paracord to do tsuka-ito a la the japanese style cord wrapping. You can do a search for him and see what he does. The method I sketched over just now is essentially the way he gets his colored synthetic cord.
I'm sure there are other makers with their own methods and favorite materials for tsuka-ito.