I've been wanting to make some natural cordage for some time so I can make a simple bow from entirely natural materials. I happened upon a large patch of tall stinging nettles so I gave it a go. There are tons of videos on You tube, so I won't bother going over the process, I'll just say, it was relatively quick and quite fun. The fibres dried over night and I used some beeswax polish on them to help keep them supple and together whilst twisting it up. I'd gathered 50 stinging nettles and used about half to make about 7' of simple cordage. I made it a bit thick, for a bow string I think 3 thinner cords made up into a string would be much better.
The end on shot of the bow shows the heartwood/sapwood and gives an idea of the belly and the waggle in the far limb, which will probably be the upper one.
There is a little twist in the limbs, E.G The boundary between heart and sapwood twists as it runs down the bow. I'll allow that wist to be present in the finished bow to maintain an even thickness of heart/sapwood throughout the limbs, I'll just need to watch for any twist or sideways bend while tillering. Mind wide limbs are unlikely to bend sideways, so it's mostly twist I'll be watch out for twist.
The Traditional Bowyers Bible vol 2 has a good section on making strings, but again there is probably plenty on You tube.