Bleeding a bonito fish is a simple process that can be done in a few easy steps. The first step is to cut off the fish's head and gut it. Next, make a small incision on the side of the fish and insert a blunted knife or skewer. Gently press down on the heart until blood begins to flow from the incision. Once the blood has stopped flowing, remove the knife or skewer and seal up the incision with some butcher's twine.