Carefully tear each sheet of nori in half, then tear each half into halves.
Moisten your hands in lightly salted water, then gently but firmly form about ½ cup of the rice into a ball. Wet your hands as necessary to prevent sticking, but use as little water as possible. Continue until all 8 are finished.
Press your index finger into the center of the rice ball and insert ½ umeboshi plum, or about ¼ tsp umeboshi paste. For variety, you may substitute minced pickles, deep-fried tofu, or your favorite cooked vegetables). Seal the opening.
The outside of the ball may be rubbed lightly with umeboshi or miso for extra flavor. Umeboshi also helps preserve the rice and aids its digestion.
Using 2 squares of nori, cover each rice ball. At first it may look a little ragged; set it down, wrap the rest, then come back to the first, and gently pack as you would a snowball.
Rice balls keep best if wrapped in waxed paper, then in paper. Do not wrap in plastic or store in airtight container.