Step 1: Inoculate about ¼th of rice with koji. This helps to create enough enzymes and strong starter culture to breakdown the starch. Some stores also keep Sake rice and rice balls, which are pre-inoculated. If you cannot find them, then one has to create a bed of steamed rice, sprinkle the koji culture on it and keep it in a warm moist room for 24-40 hours.
Step 2: Use steamed rice (not boiled rice). The goal is to gelatinize the rice but keep it firm enough for the enzymes to act on it. Boiling will make the rice too soft and the starch conversion will not be efficient.
Step 3: Cool the remaining steamed rice to 25oC before mixing with koji rice. Often the core of the rice is hotter, hence a bit of patience is recommended. Add some RO water to submerge the grains. Chlorinated and hard waters should be avoided.
Step 4: Stir the mash every 12 hours with a sanitized steel spoon. Also, check the temperature and do not allow it to shoot above 20oC.
Step 5: After about a couple of days, we can drain off the broth, filter and drink it. Adding more steamed rice will allow us to propagate the culture further.
Optional. You can dry out the sediment. It has enough yeast and koji in it to help you make a second batch.