The image above shows the basic concept:
1) with your right hand, place your thumb loosely near your right nostril, and your ring finger loosely near your left nostril.
2) Breathe in through both nostrils.
3) Place your thumb on your right nostril and exhale through your left nostril, and then inhale through your left nostril
4) Release your thumb from your right nostril and place your ring finger on your left nostril. Exhale and then inhale through your left nostril.
5) Repeat this sequence of holding the right nostril and exhaling/inhaling through the left nostril and holding the left nostril and exhaling/inhaling through the right nostril about 5-12 times
Just a note that you shouldn't try to do this is your nose is stuffed up--the nostrils should be clear (though you may need to attack a box of Kleenex a bit after--my old yoga teacher used to run around while we were nostril breathing distributing us all kleenexes, which was sort of funny.)
What struck me as neat about this pranayama is that our teacher Jacqueline's mention that this pranayama could really be done anyplace, and was good for the following:
1) when you're feeling really high energy
2) when you're feeling really low energy
3) when you're feeling anxietal and scattered
Let's face it, outside of yoga, this are my three most common states of mind, so I think taking a few moments to do this when I'm feeling low energy might make all the difference!