These days I get a SAC readout from my computer. The way I was originally taught to do it was to decend to 10 meters (33ft) write down the Bar reading on my slate, swim around maintaining the 10 meters depth for a period of time - 10 minutes/20 minutes, it doesn't really matter - write down the Bar reading at the end of the timed period. It is then simple to work out your SAC rate. You can do this swimming slowly, or swimming hard. Obviously the SAC rates will differ, but it gives a fairly real idea of the air you will use in different conditions.