Soak the beans in 10 cups (2.5 liters) of filtered water overnight (or up to 24 hours).
Drain the beans well.
Bring 1/2 gallon (2 liters) of filtered water to a boil, then add beans and reduce heat to simmer. Cook until soft, about 30 to 60 minutes (exact cooking time varies by bean).
Save the bean cooking liquid. Place a colander over another pot, mixing bowl or other heat-proof container, then use it to drain the beans and capture their cooking liquid. Place the drained beans in a large mixing bowl or put them back in the cooking pot once it has cooled down.
Let the cooking liquid mixture cool down to 100°F / 38°C.
Mash the beans, leaving about 1/4 of them intact.
Reserve 1/2 tsp (3 g) of salt for later, and stir the rest of the salt into the bean mixture.
Add the mature miso to the beans. Stir until it’s well incorporated.
Stir the koji into the bean mixture.
Once the cooking liquid has cooled down, add 2 1/4 cups (540 ml) of liquid to the mixture if using dried koji, or 1 1/3 cups (310 ml) liquid if using fresh koji.
Mix well for 5 minutes in order to allow the koji to absorb the cooking liquid.
Once the miso is well mixed, you should be able to form it into balls that hold together without any liquid coming out when you gently squeeze them. If the miso crumbles in your hand, add more cooking liquid to the mixture, stir well, and then try the squeeze test again.