Cordage is one of the most useful tools you can have in the wild and is all but indispensable in a survival situation. It can be used for (among other things) bowstrings, fishing lines, trap triggers, snares, and lashings. If you run out of paracord or don’t have any in the first place, natural cordage can save you a lot of headache.
Most people would likely despair if forced to make their own rope or string. However, the materials needed to do so are plentiful in most places, and the techniques required are actually quite simple to master.
Via Wildwood Survival
"Just about any strong, flexible fiber can be used to produce good cordage. The dried inner bark of most trees, for example, will supply you with workable raw material. It's best to look for trees with dead bark, and strip off long sections of the fibrous cambium layer between the wood and the outer bark. If you're in a true survival situation, you can even strip sections of the inner bark from living trees (it'll be easiest to do in the spring, when the sap is running) and dry them. Be sure, though, to take only a few thin strips from any one trunk (don't cut all the way around it … doing so can kill the tree). Should you have trouble separating the inner from the outer bark, just soak the strips in warm or boiling water until the fibers come apart easily.
The dried inner skin of the stalks of fibrous plants will also serve your purpose, as will fibrous leaves and even dried grasses (again, see the accompanying list). When working with pithy plants, such as dogbane and milkweed, you may be able to strip the material you need from the stalk in long ribbons. If the plant is dry, though, you'd be better advised to crush and open up the stalk … then break off short sections of the woody core, leaving a long ribbon of fibers in your hand, If you come across a supply of non-pithy plants, such as nettles and rushes, the best way to remove the fibers is by placing a dried stalk on a piece of wood and pounding it with a rounded rock. (Don't use a sharp instrument, as it could cut the fibers.) The material from annual plants will, of course, be shorter than that gathered from trees, but by splicing the fibers together, you can still make cordage of almost any length or thickness.
Animal sinew can be used to produce exceptionally strong rope or twine. In fact, a strand of it no thicker than carpet thread can hold the weight of an average man. Because of its strength, sinew is especially good for making bowstrings, fishing lines, snares, wrappings, and threads. Another useful property of sinew is that, when wetted with saliva before wrapping, it shrinks and dries as hard as glue. As a result, knotting the ends of a sinew wrapping is sometimes unnecessary. (Rawhide is very strong, too, and also shrinks as it dries.)
The longest sinew is found in the white cords that run along either side of an animal's backbone, but you can get, usable lengths from the tendons and ligaments attached to muscles and bones, as well. Simply cut out the sinew … remove its protective sheath … and clean and dry it. (When dry, it'll be very hard and brittle.) To separate the individual fibers, pound each strand with a rock, as you would for plant stalks … then put it in hot water."