Put all the ingredients in a pan over low-medium heat. Put the bonito flakes in a 'tea bag'.
When it comes to a boil turn down the heat to low, and simmer slowly for 5 minutes. Turn off the heat and leave until cool.
When the liquid is cool, take out the kombu seaweed and bonito flakes, and transfer the liquid to a clean container. Dilute by a factor of 2 to 3 when using.