It basically needs prepared like line flax.
Cut and dry, stook, then soak and rett, then stook and dry again and then break and comb.
I know that somefolks manage it just from retting (soaking it in anaerobic water) but tbh I find I lose too much and the stink clings like the worst stench.
If you break out loads, as Josh did, then you can speed things up, but you'll not make enough to make any sizeable bit of cloth from it that way; it's just too labour intensive.
Flax will give about 11 or 12% fibre from it's dried weight. Nettle will give 1 or 2%.
The staple length of nettle is much shorter too. Maybe 10 to 12cms at best, while flax can be 70 or 80cms long.
Thing is though, flax needs good arable land, while nettles will crop anywhere there's nitrogen......like the back end of a byre with the run off from cattle, or near a cesspit, basically giving something useful from otherwise uncultivated land.
It also makes a beautiful fibre and a really soft hard wearing wonderfully good cloth
Very white right from the start, unlike flax which needs bleaching, or time and exposure, to whiten it.
Be interested to see if you do make some
but if you get the chance try sowing a wee patch of flax somewhere too and try working with them both. Just sow it thickly so that the crop comes up straight and stretching.
cheers,
Toddy