“Cordage” as in ropes and cords. Stuff to tie things with. The explosive you’re thinking about is called “cordite” (or, possibly, you might be referring to detcord, which is a cord-like explosive).
Anyways, we’ll start of with gathering and processing materials. You can use just about anything that is fibrous. I can’t tell you what will work, and what won’t. You’ll just have to experiment for yourself. One possible material is the inner bark of trees (such as cedar, birch, and willow). One wait to harvest the inner bark is to find a fallen tree and remove the bark from it. To do this, use a cutting tool to make a cut in the bark down the length of the tree. Then, using a pointed stick (one with a flat “chisel” tip works best), lever the bark away from the tree, trying to keep is all in as large a piece as possible. Along the way, you’ll run into knots in the wood that will make it harder to get the bark off (you’ll also learn why wood lice have their name. To get around the knots, just place your stick between the bark and the tree near the knot and just lever the bark off the knot.
Once, you’ve get your piece of bark, lay it out in front of you, outer side down. The inner side of the bark, may have different colors. These may be white and wood-like, golden yellow, or dark and red. The dark and red is where the inner bark has begun to rot. Don’t use this. It’ll make for weak cordage. Just throw it out (not that there may be useful bark underneath the rot, so don’t throw the whole piece out. Just pull the rotted piece off like you normally would, but throw it in the trash). The golden part is where the bark has “retted”. This is where the proteins holding the bark together have broken down, but the bark has not yet begun to rot. To pull a piece off, just pry it up starting from the edge (alternatively, you can cut a slit in the inner bark and use the cutting tool to pry it up and get it going). You should try to get a piece that is as thin (a few millimeters thick, at most), wide (but probably not more than about 3 centimeters), and long as possible. Pull as must of this off as you can find and roll it up (you can work on making cordage out of it during the rest of the year. The native Americans used to spend the winter making cordage. As I said before, once you learn how to do it, you can even do it in the dark). Note that the pieces will dry over time, so if necessary, you can soak the in water for an hour or two. In any case, once you’ve started to work on your cordage, you need to split the fibers. To do this, just pull them apart with your hands. Try and make the pieces the same thickness and keep the desired product in mind (larger pieces, or bundles of fibers makes larger ropes).
The first technique is call “two-ply reverse-wrap cordage”. There are two techniques. For the first technique, choose several pieces of material that are about the same thickness as each other. Taking, one of the pieces, hold it between the thumb and forefinger of your off-hand a little bit off from the center (so that if you were to fold it in half, one end would be longer than the other. This is important to avoid weak spots). Taking your dominant hand, twist the material clockwise until it begins to kink in on itself forming a small loop. Next, while keeping the material twisted with your dominant hand, take a hold of this loop with your off-hand, pinching where the material is twisted and hold the material so that the two ends are towards your dominant hand. Next, using your dominant hand, grasp the part of the material that is farthest away from you about a centimeter or two away from where you are pinching with your off-hand using the thumb and forefinger of your dominant hand and give it a single full twist clockwise. Next, while keeping the material twisted, rotate both parts of the material one twist counter-clockwise so that the part of the material that way towards you is now away from you and advance the pinch of your off-hand to pinch the newly twisted part. Repeat the previous two step however many times it takes to make the cord. Now, along the way, you’ll likely need to splice in more material. To do this, when one of your plies starts running out, place another piece of material next to it, and just perform the two steps listed above as normal. Try to avoid splicing in material on both sides at once, though. This can create a weak point in your cordage. Also, note that if you down secure the end of your cordage somehow, it’ll eventually unravel (I’ll leave you to figure this one out yourself). The second technique is exactly the same as the first, but all of the directions of the twists are reversed. Experiment and choose whichever one of the techniques works best for you.
Later, I’ll see if I can make some instructions for three-ply reverse-wrap cordage.