So I said I'd do a how-to on onigiri many many months ago -- I'm very sorry it took me so long. :( Today I finally decided that I have to use the umeboshi in my fridge, and pulled my boyfriend into making onigiri with me (though he thought pickled plums to be repulsive and used tuna and chicken instead). I hope you like it. :)
Gather your materials. Short grain rice is best for this because it's the stickiest, and you can use anything you want as a filling -- umeboshi, tuna and mayonnaise, salted salmon, pickles... whatever.
Cover up 1 1/2 cup rice with 2 cups water. Bring to a boil, then reduce heat to low and let it stand for 20 minutes. Take it off the heat and let it cool down for about 5-10 minutes under the pot cover.
Take a small cup and put saran wrap in it, draping a good bit over the sides. You can shape it with your bare hands, but this method doesn't burn...
Wet your hands with water and a bit of salt. If you want, splash in a bit of rice vinegar and salt into the saran wrap-- either for taste, or just if you want the rice to stick together better.
Put in a blob or rice, make an indentation in the middle of it, and place a bit of your filling there. Cover with more rice.
Twist the saran wrap into a little bag and press the rice together.
Shape the ball of rice into a triangle (or a sphere, or just a ball). I (badly) drew in my other hand to show how to squish it into a triangle, since it was busy holding the camera. :P
After you're done making the rice balls, set them aside and cut the seaweed paper into strips, if you have it. It's not necessary, but it makes picking up the onigiri easier.
Wrap the seaweed paper around the onigiri any way you want, or cut it up into shapes-- and voila! You have yourself some tasty onigiri.
These are my boyfriend's first ever onigiri (before he put them on a plate). :D