Have a quart or liter glass jar sterilized take your sake and kombu and bring to boil till alcohol is cooked off.

When the alcohol is cooked off add the soya sauce mixture to the kombu mixture let boil stirring occasionally for 10 minute

Fish out the kombu add to jar makes it easier to pour liquid into jar pour contents of liquid into jar on top of the kombu when jar cools put into the refrigerator for 48 hours at least then take the kombu out