Normally, you would add kombu and bonito flakes and let those all sit together in a refrigerator for 24 hours or up to a week. But we're here to make an easy version so just mix all of the ingredients together in a bowl.
Use immediately or store. To be safe, ponzu can be stored for one month. But if you keep it away from water, you may be able to store it for 6 – 12 months.