Soak the beans overnight. Use enough water to keep the beans covered even if they double in size.
Drain the water, refill the pot with fresh water, and bring the beans to a boil. Reduce the heat and simmer until the beans are soft, about 1 1/2 hours.
Drain the beans, reserving 1/2 cup of cooking water. Allow the beans to cool down to room temperature (about 1 to 2 hours).
When the beans have cooled, sprinkle on 5 Tbsp of salt.
Place 1/2 of the beans in a 1-quart glass jar. Add in 1/2 cup of fermented black beans (or miso), then top with the remaining freshly cooked soybeans. You may need to press the beans down to get them all in the jar.
It’s fine that the culture isn’t fully mixed into the jar. Placing it in the center of the jar will ensure that the mold, yeast, and bacterial cultures will spread throughout the beans.
Sprinkle 1 Tbsp of salt on the beans, then place a weight on top. Pour the reserved water overtop so the beans are completely submerged. Leave an inch of headroom at the top of the jar to prevent the liquid from bubbling over as it ferments.
Cap with a lid that will allow CO2 to escape. I find that a loosely tightened jar lid works well.
Leave the jar somewhere dark to ferment for at least 2 months (though waiting up to 6 months is ideal).
Once you start to use the black beans, store the jar in the fridge. I recommend starting a second batch when you’ve used up roughly half the jar. That way your second batch will be ready by the time you are completely finished with the first batch.