Open the tuna can and drain the oil.
Mix tuna together with mayo, ketchup, and soy sauce.
Prepare the onigiri kit.
Make a rice mold in the triangle shape.
Place the rice inside the mold. (Don't push too hard!)
Place the mixed tuna on the rice.
Cover the tuna with the rice.
Remove the mold and sprinkle the salt on the both side.
Wrap in the seaweed sheet.