If you have an old one that you want to get rid of, it's a good idea to set up a controlled fire and try out your extinguisher on it. That way you can get some experience.

Really though, it mostly is just pull pin -> aim low (at the base and source of the fire) -> squeeze until extinguished. Sweep the nozzle from side to side to get proper coverage.

They're intended to be used by anyone with no training, so there's not much to go wrong (assuming you haven't bought the wrong type and use it on an oil fire, although most of the ones I see for sale for consumers are the powder kind, which work on anything. The water ones are the worst, and I've never seen one).