There is a chapter on bast fibers, of which nettle is one, in Judith MacKenzie McCuin's new book, The Intentional Spinner. You have to rot away (ret) the outer layers to get at the inner fibers. There's not too much detail -- she doesn't say, for instance, how long the fibers are or whether you need to combine them with something else. But it's apparently not too different from spinning flax or hemp.
BaaBaa on Ravelry