Begin by gathering all the ingredients. For citrus juice, you can use a combination of lemon, orange, and/or grapefruit juices. Also, you can also replace mirin with 2 tsp sugar + either 2 tbsp's sake or 2 tbsp water.
Now, take a sterilized mason jar and put in all the ingredients. Mix well.
Next, take the jar and steep in the refrigerator for at least one night. If possible, steep it for several days, or for up to a week. If making a big batch, you can go up to a month.
Once the steeping is done, take a sieve and drain the mixture in order to strain out the katsuobushi and kombu.
Keep in mind that you can store the homemade ponzu in a mason jar for up to 1 month but if you are replacing the mirin with water and sugar, it is suggested to use it within a week or so. Also, it is advisable to always use clean utensils when you use the sauce as it helps to avoid cross-contamination. It is important to note that the sauce will be fit for consumption for 6 to 12 months if you make sure to sanitize and keep everything clean while preparing the ponzu.