Preparation: Cut the nori sheet into three equal pieces. Remove the tarako from its thin skin. Prepare the cooked rice, a bowl of water, and salt.
Assemble: Place some rice in a small bowl, make a small dent in the center, and add the tarako. Cover with more rice.
Shape the Onigiri: Wet your hands with water, sprinkle salt (using two fingertips) on your palms, and rub together. Place the rice in your hand and gently press with both palms to form a triangle, rolling it a few times to shape.
Wrap: Wrap the rice ball with a piece of nori.
Notes
Storage: Wrap each onigiri with plastic wrap, put them in a container, and keep them in the fridge for a few days. (More details in Storage)
Japanese short-grain rice (starchy and sticky) is ideal for making onigiri. If you are unfamiliar with Japanese rice, see "How to cook Japanese rice on the stove."
The filling amount is up to you, so feel free to tweak it for your preference.