Rinse rice in a sieve for 1 minute and set aside to drain well, at least 30 minutes, but ideally 60.
While the rice is draining or cooking, whisk together the rice vinegar, sugar, salt and mirin in a bowl. Heat in the microwave for 30 seconds and stir again to dissolve the sugar. Microwave again if necessary. Be careful not to bring to a boil. You can also heat the mixture up on the stove, if desired.
Place rice and water in a pot on the stovetop (or use the rice feature on your instant pot) on high heat, covering with a lid. Bring to a boil and reduce heat. Cook for 15 minutes without removing the lid. After 15 minutes, keeping the lid on, remove from heat and let sit for 10 (but no longer than 15) minutes. Drizzle rice vinegar mixture over the top and fluff with a fork to loosely combine. Spread across a baking sheet to cool, it'll be ready for use after about 10 minutes. Always be careful not mush the rice while handling it. Sushi rice is best used for sushi at around body temperature. Place in a bowl and cover with a damp towel if not using immediately.
Notes
Mirin is a tangy, acidic and slightly sweet rice wine used in a lot of Japanese cooking. It is completely optional here, but adds a nice flavor if you have some on hand.
To use the kelp leaf (kombu is recommended), add it in to the water and rice before cooking. It adds a slight aromatic flavor for a more authentic taste.
Store in an airtight container in the refrigerator for up to 1 week. To reheat sprinkle some water across the top before reheating to keep it moist and prevent it from drying out.