Yes you can use nettles to make a tea. There are two methods that work well for this; 1. make a bokashi type ferment with fresh nettles (leaves and stems) and let ferment for at least a week then pull off the supernate and dilute 10:1 and 2. make them part of a compost heap, let this compost break down for at least 4 weeks then bag some up and brew an aerated tea, the bag works just like any tea bag.
When I use the Bokashi method I use a 6 gal. bucket with a lid, I fill the bucket all the way to the top and use a wood pounder (stick that looks a lot like a wooden potato masher but larger) to press the green materials down.
Once I have pushed all the greens down into the bucket I add some milk that is almost ready to turn, If I have been making cheese I use the whey, from there I add water to cover the green materials to about an inch (2.5cm) in depth.
Now it is time to pop the lid on push the bucket to an out of the way place and leave it alone for a few weeks.
My first time try at this My lid blew off from pressure so I have now added in a brewer's air lock which allows the gasses produced to leave with out building up pressure and it doesn't let air into the bucket.
Bokashi can be made with just about anything plant, if you add any animal materials you are going to "the dark side" and will have some serious stink happening for the first few phases of the break down.
I know one fellow that uses nothing but his kitchen scraps, including everything except meat scraps. His fermenting of bokashi usually goes for about a month before he drains off the liquid to dilute and use as a spray for his gardens. He puts the solids into his humanure compost heap.
I consider Bokashi more for nutrient addition unless I let that batch finish then add it to a compost tea to be aerated (which kills off most of the ciliates which I don't want loads of in my soil).
When I use Bokashi this way I also add some molasses to the tea batch about 12 hours into the brewing.
You could make what JADAM calls JLF (Jadam Liquid Fertilizer) by just putting the nettles together with some leaf mold and/or (worm)compost in a container. Fill it up with water and let it ferment for a couple of weeks/months. You should use this JLF with at least a 1:100 dilution.