I highly recommend removing the head and gut from the fish to reduce bitter flavor in iriko dashi. First, remove the head, and then around the belly area (bottom side), take out the gut inside (black color).
Continue with the rest of the iriko/niboshi. Discard the head and guts.
Soak the iriko/niboshi in the water for 20-30 minutes, preferably overnight.
Transfer the water and iriko/niboshi into a small saucepan and slowly bring the water to a boil.
When boiling, skim and reduce heat to low and cook for 8-10 minutes.
Remove from the heat and drain into a fine-mesh sieve over a bowl or measuring cup. Any extra dashi needs to be refrigerated and used within 3-5 days or freeze for later use.
If you don't make it right away, you can freeze the leftover iriko/niboshi and defrost to make it later on.