Add fresh water (about 1 liter/quart) and legumes to a pot and bring to a boil. Cook until "al dente", meaning a little firm, less than you would if you were eating them straight away. Cooking time varies by bean type.
Allow cooked beans to cool for 20-30 minutes.
Make a 4% brine (salt to water weight) by stirring 20 grams sea salt into 500 ml water until dissolved.
Place cooked beans in a container with a lid, then pour the salt brine on top.
Add mature pickle brine.
Cover and let ferment 3 days.
Drain beans from brine. No need to rinse them unless they're slimy, in which case, rinse briefly. In the case of chickpeas, you may wish to rub the beans while submerged in a pot of water, to remove the husks (which will float to the surface).