In a wilderness survival situation, this skill will allow you to make fishing lines, spears and arrows, and snares, as well as construct certain types of shelters. Even certain firemaking techniques (e.g. bow drill) rely on having strong cordage.
Just like fire, a good rope is a tool in and of itself.
Step 1: Get some fiber (dogbane, here)
Other fiber sources
Dead plantsThe best natural fiber sources are dead plants, though animal fur is supposedly an option. (I once saw a lady spinning thread directly off of an angora rabbit.)
Milkweed is very soft, and less allergenic. I haven't worked with it, personally, but I have seen the finished product, which looks very similar to synthetic string. The stalks should be harvested when they are dead and grey.
The inner bark from some trees is another excellent source, if you can collect enough. The trick is to find fallen branches, or dead trees with hanging bark. The best fiber trees are cedar, white basswood, tulip tree. Tulip tree (sometimes mistakenly called "poplar" or "tulip poplar") is quite common and frequently sheds branches. Tree-based fiber is strong, but coarse.
A note about cedar: You don't want the fibrous strands running along the outside of the bark -- the inside bark is where the good stuff is.
Urban sourcesPlastic bags. They're everywhere! Shred them "lengthwise", that is, in the direction of the polymer. (Make note of which direction they rip most easily.)
Back to dogbane
Dogbane grows readily in waste areas and disturbed soil, and seems to prefer partial shade. For this project, I biked over to an abandoned road that was intended for a subdivision. Plants are creeping across the roadway, the asphalt is breaking up from freeze-thaw stress and earthstar mushrooms, and there are healthy stands of dogbane, vetch, and other waste-area plants.
You'll recognize the plants by their 4-foot tall dark brown stalks and their dangling seedpods. Initially, the seedpods are paired tubes that come together at their ends but bow away from each other at the middle. As the pods decay, the tubes peel open, slowly releasing the fluff-carried seeds to the wind. (Remember, dogbane is related to milkweed.) Incidentally, this fluff is an excellent fire-starting material -- but that's a different instructable.