It all starts with the oak galls - in my case, imported ones (which was done in the medieval times as well).
These get crushed, and then they get to soak in water for a few days, with a gentle stir at least once every day.
The next step: Adding iron sulphate - that's the lovely green stuff...
...which instantly turns the brownish liquid black.
And now the whole thing needs to mature again for a few days...