Really easy to make and is a very nice quick snack. Just cook rice, sprinkle with Japanese rice vinegar to taste (and a bit of salt if you like), mix. And then mix with any of your desired seasonings or fillings. In this case, I used flaked tuna from a can, but you can use all sorts of fillings, like furikake or pickled vegetables. I've used xue cai before, and it turned out really well.
To wrap it quickly and efficiently, put a piece of plastic wrap over a bowl, and scoop the desired about into the bowl, then you can shape it and wrap the onigiri up for whenever you want to eat.
Forgive the photo quality as the steam in the plastic wrap makes it really hard to take good photos.
One cup of cooked rice can make around two small onigiris, about the size of my fist.