I`ve just experimented with this once.
I gathered the leaves and some flowers in a bag, about 2-3 litres worth.
Let it sit for a day (room temp) to "ferment".
You will see that the leaves get a little bit "floppy" when they are done.
Then I rolled a good pinch of leaves between my palms.
Do that until all the leaves are broken down. Then lay it out on a baking tray and dry it in the oven (low heat, door open) overnight.
The brewed tea is of right color, but tastes a little like green tea blended with dry grass.
Guess it is best blended with black tea.
I have read that tea in Russia in mid 19th century could contain as much as 60 percent fireweed.
Another more advanced method is to use a meat grinder on the fermented leaves.
The pulp is then dried on a drying rack.
This tea brews better/faster, and looks more like real tea.
Taste is more or less the same though.
Pictures below are tea made with just rolling the leaves, and the second is grinded leaves on the drying rack.