This is a practical way of doing it. If your altimeter is set to QNH 1009mb (pressure at mean sea level) and you land on a runway which is 90ft above sea level your altimeter will show 90ft. Now, suppose you want your altimeter to show 0 ft so you know that you're on the runway (QFE) - what do you do? You would have to wind off your altimeter. You would wind off 90ft which would be 3mb of pressure. Wind off pressure - wind off height. Your altimeter would then show 1006mb. If the airfield elevation is above sea level then QFE will always be less than QNH. If airfield elevation is below sea level QFE will be greater than QNH.
Thats all folks!