There are many myths about preparing the fibres of this plant. It is said to be hard when it in fact is very easy. No boiling or retting is neccesary. This time I will only focus on the process after the "skins", or inner bark sheet, has been harvested and dried. As there is no nettle available here now, I will cover that when it gets in season again.
The sheets are soaked in warm water (temperate). Don't break the dry fibres into a cup, that will break them, but wet them sligthly before to make them supple. Seperate the strands from eachother as that will make scraping them easier.
Then you scrape the brittle outer shell off with your nail. Support with an other finger (one without the scraping nail) and pull with the other hand. Scrape first to one side, then the other way. By gently loosening the bundle of fibres and outer bark collecting in your "off" hand when you meet resistance you can get quite long fibres.
Now the fibres are ready, that's all! The fibres are strong, excellent for strong fishing lines, lashings etc. Nettle is not very durable however. Be careful to always dry the line after fishing or it will quickly desintegrate. And by the way: The scraped off outer shell, which is more than likely to be mixed with quite a few fibres, make excellent tinder.