Hi Mace,
pick the bigger, older looking nettles (by grasping at the base firmly, to aviod the stings). After stipping the leaves (good nosh), lghtly pound the stems to break up the fibres and then you can strip the outer fibres from the inner core.
You can then strip down the fibres to the thickness you need.
Hold the end of two strips of fibre in one hand, with the fibre drapped across your upper thigh. Place you palm on the fibres and slowly roll the nettle down your leg, at the same time, twist the end you are holding in the other direction. By doing this, you should slowly twist the individual strands and, at the same time bind the two strands together.
Probably not the best explaination but much easier than it sounds.
Good luck
Baggins