Zone focusing is pretty straightforward, the photographer simply uses the DOF (Depth Of Field) effect to have the desired object(s) in focus. Zone focussing comes in handy, when there is no time to fiddle with the camera controls, or when the photographer wants to be extra unconspicious - without using the viewfinder to focus (aka. shooting from the hip).
Once you know what an f-stop is, and how to set it on your camera, you're good to go. After some time you will get better at guessing distances and you'll be a master of zone focusing in no time...
In the illustration above, the aperture (f-stop) is set to f8 and the focus is set to 2m (~7ft). The focus ring also shows f-stop markings to either side of the focus point (DOF scale).
Every f-stop shows a line directly related to a number on the distance scale. In this example everything from 1.5m to 3m (5ft to 12 ft.) will be perfectly in focus. This works at any distance and with any lens with DOF markings.
related: hyperfocal distance