Here you go buddy. This is what I posted a bit back for someone else but is probably easier to look on the sites because they have pics and stuff.
This is how I do it:
Cut the tall old nettles off as low down as possible, not dead ones, still green.
Strip of the leaves
The nettles consist of a kinda hard woody hollow stem and the fibres you want are on the outside of this so lay the nettles on a log and squash the nettles with a stick to 'pop' the stem. You don't want to damage the fibres.
Open the stem out, like you have just cut a slit up the side of a toilet roll opened it out. (got me?)
Now you have a strip of nettle with the fibres on one side and the hard pith on the other.
Snap the pith but not the nettle fibres and pull the pith from the fibres. You want to try to get the fibres as long as possible and clean of the woody stuff inside and old stuff on the outside. Sorry I'm not deliberatly trying to bore you!
Dry the fibres in the sun or radiator. These are the raw fibres and are ready to be rolled into string.
Its quite hard to explain the rolling so you could try plaiting the fibres or roll them. Take a few fibres and fold it not halfway, otherwise the joins will be together later. The couples of fibres you choose will be half the string thickness.
grip the two halfs of the fibres near the fold between thumb and index finger, hold the fold and twist each half together but indivually. Let go of the fold and let the two strands twist together like string- magic!
Grip the new string and trist the fibres again. The fibres need to be twisted the same way and the string twists in the other. Try to keep the fibres equal thickness and when the fibres need lengthening add in some more nettle.
I think ray roll the fibres on his thigh and that is possible if the fibres are quite thick. This roll is how I make cordage from other sources aswell, such as clematis bark.
It really is easier than it sounds when you get started and you develop your own techniques after a bit.
Any bits you don't understand let me know
cheers