I take a 5 gallon bucket and fill it with weeds or even some grass clippings. Deep rooted weeds like dandelions are supposed to be better because the plants pull up nutrients from deep underground that are depleted closer to the surface. Fill it with water and let it ferment, the longer the better. I started my bucket several months ago.
For me the mess starts bubbling after a few days, sometimes so much that it tries to push the weeds out of the top of the bucket. Strain some out and mix it one part tea to 10 parts water, more or less, and pour it around your garden plants. I usually water it in after the fertilizer has had a chance to soak in a little.
The theory is that the fermenting weeds release minerals, nutrients and nitrogen into the water. Whether or not it's true or really works, I couldn't say for sure, but I believe the theory of it is sound.
And if it works, it's free organic fertilizer! Can't beat that. I'm pretty sure it helps my plants.
that's the only thing people used in the past (besides manure). they grew veggies on the same plot without any issues and didn't destroy soil.