If you have home switches fitted you home the machine when you start, this then lets Mach know where the machines 0 position is for each axis. You can then Jog each axis to where you want to start your work (G codes zero position) and then just zero each axis by pressing the button at the side of them. This will now be your work offset zero position and you can start cutting from there. The actual Machine zero is still at your home position, its just that you have now told Mach that the new zero psition is an offset.
Hope this makes sense.
Hood