Rinse the dried soybeans well and place in a large bowl. Cover with 500mL of water (2 cups), and leave overnight on the counter. After 18-24 hours have passed, drain the soybeans. They should have doubled in size.
Cook soybeans by placing them in a pressure cooker with the remaining 500mL of water (2 cups). If using an Instant Pot, set timer for 20 minutes at high pressure. Let the pressure vent out naturally, otherwise you will get a lot of bean juice steam on your kitchen counters. Alternatively, you can boil the beans and water on the stovetop for 45 minutes or until the beans are tender but not have fallen apart.
Meanwhile, clean your wide-mouthed glass jar or crock with hot soapy water. Turn upside down and leave to air dry.
When the beans are cooked, drain them. Reserve 125mL (1/2 cup) of bean juice liquid for later. Note: Use the beans quickly after cooking to prevent unwanted bacterial growth. As well, the cooled bean liquid tends to sour quickly.
For a smoother consistency, you can mash the beans with a food processor. For a coarser consistency, you can use a potato masher or your hands. Make sure to wear gloves if you are using your hands to mash the soybeans.
Soybeans should be at room temperature or at least less than 40°C before mixing other ingredients in. In a large clean bowl with gloved hands, mix the mashed soybeans with 350 grams of dried rice koji and 85 grams of salt and 3 tablespoons of miso paste (optional).
Add 2 tablespoons of the reserved bean liquid to create a looser paste, and to help rehydrate the rice koji. While you don't want too much water in the miso as it increases the chances of unwanted microbial growth, feel free to add more bean liquid to create a looser paste.
Pack the miso paste into small balls that are 1-2 inches in diameter. This will help to ensure that when packed into the jar, the miso has no large air pockets in it. We want the miso to be in an anaerobic (oxygen-free) environment.
Dip a paper towel in the neutral alcohol and use it to wipe the inside of your container. Pack the miso paste balls into the container, pressing down to ensure that there are no air pockets.
Put the container on top of parchment paper and use a pen to trace the outline of the container. Cut out the parchment paper and place in the container, pressing it to the top of the miso paste.
There will be a gap between the container wall and the edge of the parchment paper where the miso paste is exposed. Sprinkle coarse salt on this exposed miso paste.
Use a clean paper towel dipped in alcohol to wipe off any excess miso paste that is above the parchment paper in the container.
Now, you'll need to place something heavy on top of the miso paste. Fill an empty bag with salt or rocks. Tie it up and place it in the container directly on top of the miso paste.
If the container you are using is glass, place the container in a bag so that sunlight cannot penetrate it. Close the lid of the container. Make sure to label the container with the date so that you can tell how old the miso paste is. Set aside.
Check the miso paste in 3 months. Remove the weight and see if there is any brown liquid on top of it. This liquid is called "tamari". This liquid byproduct of fermentation has a nice umami flavour which you can use as a seasoning in your cooking. If you don't see any dark liquid forming on top of the miso paste, put something heavier on top. The weight on top is needed to keep the environment anaerobic for the miso.
Leave the miso for another 3 months and taste. The rice koji should have softened and the colour should have darkened. If the miso hasn't developed much of a flavour, leave it to ferment for another 3 months and check again. When you are happy with your miso, place it in the fridge to keep long term.