Use a rice cooker. When the rice is ready, transfer the rice to a large bowl and add in 1 tbsp of salt and mix well.
Make Kombu fillings
Soak the Kombu in water for 15 minutes until it is soft. Cut the Kombu into thin, small slices and put back into water.
Add some rice vinegar to the water and let it soak for another 15-30 minutes, depending on how hard the Kombu is. Rice vinegar will help it become more tenderised.
Place this mixture into a sauce pan on low heat. Add in the sake, mirin, soy sauce, sugar and bring it to a simmer. Cook for another 20 minutes. If the mixture starts drying out, add in a little water.
When the kombu is done, it should be soft, well-caramelised with no liquid left in the pan. Let it cool on the side.
Creating onigiri
Use a damp cloth to wipe your onigiri mold and its top piece.
Fill the rice up until halfway and make sure to press it down to all corners of the mold.
Make a deep indentation at the centre of the rice and add in the filling. Be careful not to overfill.
Fill in the rest of the mold with rice. Use the top piece to gently press and mold the shape.
Remove the onigiri out of the mold and take a nori sheet to wrap around it. Continue to make more onigiri until you have no more rice.