To make homemade miso, soak 8 cups of soybeans in water overnight. Though miso generally contains fermented soybeans, you can substitute other beans, such as garbanzo, pinto and black beans. Cook the beans in a saucepan until they are soft, and strain and reserve the cooking liquid.
Allow the beans to strain for a few minutes to thoroughly remove excess moisture and cool the beans slightly. Add 1 1/4 cup of sea salt to 1 3/4 cup of the cooking liquid. Process the beans in a food processor, or mash them by hand until you have your desired texture. Mix 4 tablespoons of unpasteurized miso paste into the salted liquid. Add the mashed beans and 4 cups of koji, a type of grain with a white mold injection.
Thoroughly mix the ingredients until they create a thick paste, adding additional cooking liquid if necessary. Wet the insides of a jar or fermentation vessel, place salt inside, and shake the vessel to coat the sides. Tightly pack the miso in the vessel, pushing out any air. Cover the miso in a layer of salt, and place a heavy weight on top.
Secure the container with the fermentation vessel lid, or cover it with cheese cloth and a towel, and secure the top with a rubber band. Label the container, and store it in a cool, dark place for one year.