Place the rice malt into a plastic container and break it up by hand if there are lumps. Add salt and firmly close the lid, and shake the container to mix.
It's not a big deal if you add too much water on the first day. Pour in the measured water slowly, just enough to cover the rice malt.
Cover with plastic wrap, close the lid, and store at room temperature.
The rice malt will swell from soaking the water by the second day. Add just enough water to cover the rice malt completely, and mix.
It seems better to mix it daily, but you can mix it once every 2-3 days and that is OK, too. If the rice malt and water separates, mix well.
It is done once it has thickened most of the way through, about 1 week-10 days. Remove the plastic wrap once it is done, cover with a lid, and store in the fridge.
This is the rice malt I used. It is already broken up, so it is easy to use. If there are any lumps, break by hand and mix.
Once you have used up 1/3-1/2 of the shio-koji, make a new batch in a separate container and then add that into the old batch. The flavor will become deeper and more complex.
Story Behind this Recipe
I found that shio-koji used to be made with 540 ml of salt added to 1800 ml of rice malt, so I made some for storing. The taste was rich, so I have made it like this ever since. Mold grew in the shio-koji that I made before, so I increased the salt a bit, and since then I haven't messed up.
I reduced the amount of water in the ingredients list to avoid adding too much water, so add just enough to cover the surface of the rice malt while keeping an eye on it. If you add lots of water on the first day, you don't need to add water the second day. You don't need to add water after the second day.