Rinse the rice, and add soy sauce, sake, water up to the usual level in your rice cooker. I recommend using a little bit more water than usual.
Then, just cook the rice as usual! If you have any rice left over, make it into onigiri (rice balls), and have them as a night-time snack or for breakfast.
Story Behind this Recipe
I learned that this rice is a Shizuoka-only thing for the first time when it was featured on TV! I made it for the first time in ages. It's a very nostalgic dish for me.
Whenever we had a town potluck, I always made this. I always made it by eyeballing the ingredients so it has a slightly wet texture, but that was actually quite good, so I think you can cook the rice with a bit more water than usual. Adjust the amount of soy sauce and sake to taste.