Comfrey is a great bio accumulator and will search out really deep down nutrients
With an average dry leaf NPK is 1.8-0.5-5.3. comfrey also contains calcium.
So anyways I have bocking 14 in the garden so that's about the hardest thing to source but no doubt you'll wander past some once you know what your looking for
Other items;
Rain water
Molasses
Microbial culture ie LAB or EM1a
A container like a jar or bucket
As I do think I'll be needing vast amounts I've just been using pasta sauce jars
Method;
Collect healthy young growth with no signs of deficiency or pests. You'll only need about 20% the volume of your planned container
I think chopped the leaves to increase the surface area for the microbes to get to work
I went with a tea spoon of molasses for 500ml vessel
Then the same volume microbe inoculant as the molasses
Then just stir till the spoon is clean and label
Leave the lid slightly loose for the first 7 days and after those 7 days give a lil stir and stand for another 7-14 days. Filter off the remaining plant material (feed this to ya worms or compost bin) and your good to store for 6-12 months
Application rates are generally 2ml per litre for soil drench but different crops will vary