Soak 2 cups soybeans in water overnight
Drain and boil the soybeans until soft
Mash the cooked soybeans into a thick paste
Mix the soybean paste with 2 cups roasted wheat flour
Add a starter culture of koji mold if available
Spread the mixture in a clean tray and cover loosely
Keep it in a warm place for several days until mold develops
Transfer the fermented mixture to a large jar
Add 4 cups saltwater brine
Stir well and cover the jar with a cloth
Let it ferment in a warm place for 3 to 6 months
Stir occasionally during fermentation
Strain the liquid through a clean cloth
Boil the strained liquid for a few minutes
Cool and store the soy sauce in a sterilized bottle