Soak the dried soybeans in 6 cups of water overnight.
The following day, drain the soybeans from the liquid they soaked in.
Place the drained soybeans in a heavy-bottomed pot, and cover with fresh water by 2 inches.
Cook the soybeans on medium heat until they are soft and can be easily mashed, roughly 3 hours.
Once the soybeans are fully cooked and tender, drain them again and let them cool.
Once the beans have cooled, mash the beans to a paste consistency, or process using a food processor or immersion blender.
In a food processor bowl, combine the koji, warm water, and salt together.
Process the mixture in a food processor until it reaches a paste-like consistency.
In a large bowl, mix the blended soybeans and the koji and salt mixture together.
*The paste should form a stiff mixture that can hold its shape when squeezed into the shape of a ball. If you find your mixture is too dry and crumbly, add a bit more water.
Clean and sanitize a mason jar.
*If you don’t have sanitizer solution, just make sure your mason jar is incredibly clean, washed with really hot soapy water and rinsed thoroughly.
Pack the miso mixture tightly into a clean glass jar, pressing down as you go along to ensure there are no air pockets.
Leave about 1 inch of space at the top of the jar.
Cover the whole surface of the miso with a very thin layer of salt.
Cover the salt surface with a layer of plastic wrap, ensuring that the plastic wrap is pressed down firmly against the paste.
Weigh the layer of plastic wrap down.
*To weigh down the plastic, I like to take a Ziplock plastic bag and fill it with something like white granulated sugar or salt, and place that plastic bag on top of the miso paste to weigh it down evenly.
Cover the jar with a piece of clean cloth, and secure with a rubber band and loosely-fitting lid.
Store the jar in a cool, dark place for at least six months to allow the miso to ferment.