Yaki-ago dashi has a clear elegance and deeper flavor because flying fish has a low proportion of fat and more protein. It has less fishy odor. Other characteristics of yaki-ago dashi is it has a slightly savory aroma and sweet flavor.
Remove the heads and internal organs of the yaki-ago. You can still make a proper taste of yaki-ago dashi without taking the head and internal organs. But if you take it, you can avoid the off-flavor (bitterness or other unpleasant taste) being formed in the dashi. In this step, lightly toast the yaki-ago in a frying pan, it'll be more delicious.
Break the yakiago into small pieces, soak in water in a pot/bowl and leave for half a day. If the room temperature is high, you can put it in refrigerator.
Put the pot on a low heat, and turn off the fire when the foams start to rise.Don't boil it until big bubbles are forming, because it will make the dashi smelly.
Drain the yaki-ago with cloth, kitchen paper, or strainer.