It is really important that all equipment is sterilised before use. Amazake is normally made at 60°C, which helps to prevent dangerous bacteria from growing. Cold brewing is much more likely to result in bacterial growth, so special care must be taken to ensure everything is spotlessly clean before making amazake in this way. I boiled the jars/bottles/spoons in water before use. Other methods of sterilisation are good (eg. commercially available sterilising solutions), but double check they are safe for use on metal/plastic, if that is what you're using.
Add the koji, cooked rice (any type of rice is fine, see notes below) and boiled water (which has been cooled to room temperature) to each bottle (the amounts can be scaled up/down for different sized bottles) This can be quite fiddly for small necked bottles - like the ones I used! I recommend using a wider necked jar.
Store in the fridge for one week.
The amazake is now ready! You can drink it as it is. FIlter the rice out for a clear, crisp drink that is delicious cold. Blend the rice in, for a creamy drink that is delicious hot. Or experiment with adding different flavours - eg cinnamon to hot, creamy amazake or fruit syrups to cold filtered amazake. Enjoy!