All of us at one time have fired a rubber band with the original rubber band gun, the hand. While there are many methods of doing it there are some favourites.
Hand Pistol - the original and still the best
Make a gun with one hand (a fist with the index and thumb sticking out). The first step is to loop the rubber band round you little finger like so
Next we pull the rubber band taught and pull it round your thumb like so
Now put the stretched end over your index finger that is sticking out so it looks like this
Now to fire, straighten out your little finger and the band will fly off in the direction your index finger is pointed.
Multiple Band Variation
This is the same as the previous one with one or two additions. After you have done the above, repeat it again, but put the second band under the ring finger. And again with the middle finger and another rubber band if you want too. Now to release, lift each on of the fingers in reverse order (the last one you put on comes off first). This takes a little more skill but with a bit of practice you can have yourself a nice little three shooter.
Tim Morgan has gone to extremes and produced a huge guide to firing the perfect rubber. Have a look here.