When you have to use an elbow crutch, ideally a physiotherapist should teach you how to use it. However, sometimes you may have to learn it yourself (like I did now). Since there isn't much information available online, I thought I'd post a HowTo. Remember that I am in no way authorized to write about this; I am writing whatever I learned by myself.
Hold the crutch on the hand opposite to the weak/broken leg. I have a fracture in my left leg, so I am holding the crutch with my right hand. (Ensure that the crutch is of the right height; you should be able adjust its height to suit you.) Move the crutch and your weak leg forward. Applying pressure on your weak leg and the crutch, move the other leg forward. That's pretty much it.
I made two mistakes. I walked too fast. My knee started hurting after some time; so now I'm taking it easy and slow. I kept looking down as I walked. For some strange reason I don't understand, looking straight makes walking easier. Hope this helps. Wish you a speedy rehabilitation :)
Update (Feb 9, 2009): I learned a couple more things today. Breathing smoothly makes the walk less tiring. Holding onto objects (like tables, etc.) for support is very good, but not if it would take away your confidence when you have to leave them behind. Look carefully if your mind says "oh my god" when you leave such a supporting object. Confidence is very important than anything else.