When I have onigiri for lunch on a workday (I'm still on holiday now), I usually make it part of a bento box and use a partitioned container which includes fruit and perhaps a little sweet thing. I can get quite fancy with my packed lunches, you know. After all, one needs something to look forward when one has to drag oneself to a cold, uninspiring office space.
I cook one cup of raw rice, and that's too much for me to eat in one sitting, but since I use a rice cooker, it's just more efficient with that amount (the rice can also be cooked on the stove, of course). If each ball is a tablespoon of rice, I can probably get around 15 servings.
After the rice is cooked, I leave it covered in the pot for about 10 minutes. In the meantime, I heat up about 2 tablespoons of rice vinegar and dissolve 1/4 teaspoon of salt and 1.5 teaspoons of white sugar in it, then mix the vinegar through the warm rice.
I make balls using my plastic contraption, but it can be done by compressing the rice with the hands (wet them first so the rice doesn't stick). Oh, I also cut nori into strips (I simply cut along the perforated lines in the seaweed sheets) and wrap those around the balls, then place the salmon on top. Serve with Japanese soy sauce and wasabi.