Pour soy sauce and mirin into a small saucepan over high heat.
Bring it to a boil and then turn the heat off in order to evaporate the alcohol content of the mirin.
Add kombu strip and bonito flakes to the saucepan while the mixture of soy sauce and mirin is still hot.
When it has cooled down, add a citrus juice of your choice.
Place it in refrigerate overnight.
Strain the ponzu sauce with a fine mesh strainer and pour it into a sterilized jar.
Notes
*1 If you want to make it Gluten Free, use gluten free soy sauce. *2 If you want to make this sauce vegan or vegetarian, you can omit this ingredient. *3 Use any citrus sauce you like but see the tips in the above post to balance the acidity to your liking.