Sake is a difficult thing to make. I'll post a simple recipe for 1 gallon but highly suggest checking out the following site:
1 gallon Sake
1 2/5 cups Koji
5 3/4 cups Rice
9 1/10 cups Water (Good mineral/spring water only like Fiji water)
Reading the site listed above Taylor explains that traditional Sake is made up of the 3 above ingrediant (Plus Yeast) in the following ratio (Koji:Rice:Water) 25%:100%:160%.
I am sure there are measurements that are better to go by than cups but I just know 1 gallon of water is 16 cups so applied the above ratio to get close to 16 cups.
I have seen other people do 1 gallon Sake batches with the following though:
1 cup Koji
4 cups Rice
Water to 1 gallon
A lot of people say to add citric acid or acid blend but that should be highly avoided because the Koji will add a bit of sour to the mix to balance out any sweetness that is left over. Adding any more acid will make the drink too acidic. Especially if you go with the second recipe which will be more dry.