What you are hoping to make is nettle tea. There's no set formula as far as I know .. just cut as many as you can and jam them down into a bucket or drum, then just cover them with water and leave them alone for two to four weeks.
You will know when they are ready because the smell will take your breath away. Get a watering can and dilute a cup full of the liquid with the rest being water and feed your plants with it. That's how I have done it in the past. You can do,the same with seaweed.
It seemed to work, the only issue is the smell when handling it.