Picking nettles for tea.
For tea, I pick nettles (Urtica dioica) up to 40 cm tall, and bundle to dry.
Put on your gloves, make 2 bundles of about 7-10 stems each, equally long, and tie one bundle at each end of a shortish string. Hang over something suitable (I have a line strung across a shady room), let dry for 7-10 days (the thickest stems don't bend, they break). Put on your gloves again, cut off the string, pull down your bundles, put them on the floor in the middle of a largish sheet, and remove the strings. Take a good grip of the thick end of the stem and pull the leaves off, all in one go. Put everything into a glass jar, close lid, label.
The floor: things will fly all over the room, and they won't fly as far if you're on the floor.
The sheet: it's impossible to get all the goodies of any given plant straight into the jar, if it's on a sheet you get pretty much everything.
These nettles will still sting.
After the nettles have grown too tall you can just pick the tops:
Pick the top 4-6 or even 8 leaves of nettles, pour onto a sheet onto a layer of newspaper to dry (or boil'n'slice, for stew).
When nettles get too tall they're also approaching the "too old" line, but the topmost leaves are young and nice. The stem is very stringy, but again, not at the top.
You can't pick nettle tops once the plant starts to flower. Or, well, you can, but they're not at their best anymore.
If you've missed both the "under 40 cm tall" and the "tops before flower" nettle picking times you can take a scythe to your nettle patch and wait for the next crop of fresh young nettle, 2-3 weeks later. The taller plants you've missed with your scythe will take your revenge on you, though, and sting something fierce.