Bearing in mind that I'm no real survival nut, :wave: (sorry guys) but I make lots of different ropes and twines by hand....I find barks are best used for twine having been stripped off the branches and soaked for a bit....find a bucket if you've got little bits or a burn or a loch if you've got loads...this makes it an awful lot easier to seperate the layers....the exception is that short space of the year when the sap is rising and you can use it pretty much straight away, especially willow and hazel, line and elder. Then it's simply a matter of how carefully you clean it up, taking off the outer, hard, break-upable bark and keep the inner stuff unmangled by clumsy knives, that dictates the quality you get. Two strand is really straight forward, three strand starts to need digital acrobatics, but is fine, four??? find a twister and a couple of friends.
Think of it as, "twist and lay"
You put the twist in to however many strands you are aiming for, all in the same direction, but you lay them down in the opposite one. Subsequent twists bind them in place and create a flexible twine/thread/rope. If you can make a grass rope, then the principle is pretty much the same. If you have really good lengths then you can twist them seperately and then roll them together on the alternate spin.
e.g. twist clockwise, lay anti clockwise or vice versa.
Somewhere there's a series of photos of me making willow bast twine and then using it to make a Neolithic knotless net when I was working at the Crannog. I'll see if I can find them. It's a brilliant idea, because you can undo your net and still have your rope but with a modern triselisk net knot you can't, you've either got rope or net.
Toddy