I found very simple onigiri instructions on justbento.com But, in short, take Japanese (sushi) rice, which is rather sticky, and make it into a ball with something tasty in the middle. I made 1 1/2 cups of rice and ended up with 12 pretty large onigiri.
To shape my onigiri, I used a rice mold that I bought at an Asian grocery store for a few dollars. Onigiri can come in many different shapes. Mine are fat logs. If you don't have a mold, you can just make them into balls.
Finally, I decorated my onigiri with nori (sushi wrappers) cut into strips and sesame seeds. A sprinkle of salt on each one is also traditional and a real must if your filling isn't too salty.
I'm pretty psyched about my onigiri. They were quick to make and the variations are virtually endless. And they freeze well. I brought a few to work in my bento box for lunch this week - they reheat well in the microwave or are fine eaten at room temperature. I'll definitely be making more soon.