Onigiris are plain Japanese rice wrapped in Nori Seaweed. Unlike sushi , they are not flavoured with vinegar. The rice can be made in a rice cooker, steamed or on a stovetop.
The key to great tasting rice is to make sure the amount of water is correct. When using short grain rice, the amount of water should be roughly 10% more than the weight of the rice. Sushi rice is a much stickier than jasmine rice, which makes them great for onigiri.
The rice should be made the day it is consumed. Rice that has been put in the fridge tends to get hard, so if you really need to make this ahead of time and you might want to slightly steam or microwave the rice before wrapping them. Onigiri are meant to be eaten at room temperature, so it's best to let the rice cool down if it's steaming hot. Otherwise the nori will get soft.