Steps: catch fish. Brain fish to stop stress hormones from releasing into the meat. Clip the gills and make a slight cut to the tail to make sure the flesh is as clean as possible. Blood in the flesh will diminish the flavor of the meat. Knowing this will help you bring the fish to its optimal deliciousness. With small fish, you will want to age it 1-3 days for sashimi with the exception of bonito, which is best consumed the day-of. The rigormortis process will allow the flesh and the sinews to break down and become extremely tender.