If you are using as a marinade, mix the ingredients in a big ziploc bag, throw in whatever you're marinating and turn the bag to coat. I like to marinate salmon for at least 30 minutes, although overnight is best.
If you are using just as a sauce to mix into onigiri filling, you don't need to cook it. Just use it as is. But if you've marinated meat or fish in it, and then you want to use it as a sauce, cook it for a few minutes first on the stove or in the microwave. Easy peasey!