The difficulty with the nettle is extracting the fibre from the stems. Traditionally this has been done in the same way as Flax and Hemp, i.e. with water or dew retting. This is a long and strenuous process, but a wonderful challenge, and there are no rules, unless you are an archaeologist trying to imitate historical methods.
With no rules, but inspired by literature written in the past and now and experimenting for 10 years, I have come to the conclusion, that if you are doing all the processes yourself, or with very little assistance, the simplest way is to root ret the nettle stems and cottonize the fibres (to card and spin short fibres in the same way as cotton).
You may not get fibres quite as fine as if you dew or water ret, but, the fact that you don’t have to handle the nettles while they sting, or while they are rotting and smelling, not to mention the use of water and the pollution of the waste water makes producing the fibre in this way worthwhile.
As a bonus the waste straw also has many uses.
Retting is the process of decomposing the pectin in plant stems by fermentation in order to free the line fibres from the woody matter