Making good cordage is, like most primitive skills; easy, but practise is required. It can be made from virtually anything fibrous, from paper to rawhide. But as any economist would tell you: Don't have the money in the bank, when you can make more on stocks. In other words, there is no reason to use the inferior fibres of rosebay willowherb, when the nettle is equally abundant and gives a better result.
There are several types of cordage. There is the single strand cordage. This is the fastest method, but usually only usable for static tasks, such as wrappings. Roots are usually used this way. Rawhide is an exception to this rule, internally it is really consisting of multible fibres and a very strong glue, making it behave like regular cord even if it is single stranded. Single stranded cordage have to be spliced with knots, while twisted and braided cordage can and should have fibres added continously.
Twisted cordage is the second fastest to make. It is stronger than single stranded due to internal friction and the alignment of the fibres gives an artificial stretch to the fibres. Useful in bowstrings to absorb excessive shock, but often not desirable in fishing lines as it reduces the feeling with the fish. It can have two plies or more, but making more than two is messy and I almost never do it.
It is done by either thigh rolling or by holding the other end firm (in the mouth or in an other way) and twisting with both hands. Thigh rolling is way faster, but the results are less even and thight, at least for me. Also, it is hard to thigh roll more than two strands and if the get too thick. But for long lenghts of cordage, for instance for fishing nets there is really no other way. To achive sufficient friction surface to roll on, either wear no trousers or wet the area you roll on first.
Working with the fibres wet is sometimes advantageous, regardless of method. The animal fibres I feel however become too limp. Some people like it, but I don't. Experiment on your own to find what you prefer. To make sure the twisting doesn't cause the finished cordage to untwist further down you have to have some means of relieving the twist. No problem with short bowstrings, but with longer cords a something is needed to wind it up on. A spindle with a wheel is excellent for this and very high on my "to make" list.
Every girl and most boys knows how to braid a simple three stranded braid. Braiding is stretchless and thus good for fishing lines. It is extremely slow to do however, and the thinner you make them the slower it is. It is also possible to braid with four strands, but I have yet to learn that. Below is an illustration of twisting and braiding.
Knowing how to make strong cordage is essential for the primitive. I have just started thigh rolling some 1000 metres of nettle cordage for a new fishing net. We'll see how much I end up making, but for now my goal is 1000m. To the left on the photo is braided fishing line and to the right is thigh rolled cordage for nets. For scale: The fishing line is approximately 1,5mm thick.