I'm so glad I found this thread! My crafty friend heard about nettle yarn and told me aboutbit and now I'm taking some baby steps in play with them.
I just harvested a bundle - maybe 20 or so stems - last week. They were about 4 feet tall and sun-grown. Still early in the season too. Having not researched fully beforehand (I browsed this forum and watched the Russian man's amazing video), I dried them on our metal roof. I think in the future I'd go for processing green, though I'd like to try dew-retting and we have a beaver-dammed stream that might be perfect for water retting.
I'm a little nervous about how to proceed with this batch, but then again it's not like I have a scarcity of nettle plants to work with!