Put the amazake and water in a wide-mouthed jar, then store it in a warm place that maintains a temperature of about 30°C.
Refresh the air once a day by opening the lid. Lightly shake the jar.
After 2 or 3 days, it will start to bubble, indicating that it is finished. Another sign of complete fermentation is foamy liquid and floating rice grains.
It will smell like beer or alcohol when ready. If it tastes slightly acidic and carbonated, it should be prepared for leavening.
Story Behind this Recipe
If I use dry yeast or baking powder to make sake manju, I'm not being true to its namesake...that's why I decided to make amazake to prepare the leaven. I also use the leaven when making anpan (anko-filled bread). I suspect it will also taste great in crusty bread, but I still haven't tried it. Some day...
Although many sterilize their jars by boiling them in water or filling them with boiled water, I don't. Feel free to do as you please. I have never done it to eliminate any risk of spoilage.