Use scissors to cut fireweed stalks just below the last pretty looking leaf; fireweed leaves can get scraggly lower on the plant. Strip the leaves from the stalks and set in a bowl.
Grab a few leaves and using your palms, roll them vigorously together. You want to roll them into a fairly tight mass, bruising the leaves. Set the bruised rolls into a container that has a lid.
When you have all the fireweed rolled and bruised, put the lid on the container and set out of direct sunlight at room temperature. Let this ferment for 2 to 3 days, moving the leaves around once or twice a day. You want an environment that's humid so the leaves don't dry out but not so humid they all mold. You will just need to watch it. Discard any moldy leaves.
After the leaves are a very dark green, almost black -- two to three days -- lay them out in the sun to dry if you live in a hot, dry place. Other ways to stop the fermentation are to steam the leaves for about 5 minutes or roasting in a 350F oven for 20 minutes. I like the sun-dried and steamed methods. If you do steam, you'll then need to dehydrate the leaves anyway, so a steam bath followed by a hot period of sun drying makes the best tea.
Store your tea in a Mason jar in a cool, dry place. I like to add a silica packet in there to prevent molding. The tea will keep up to 2 years this way.
Brew as you would any normal loose leaf black tea.