BD may have said this already in a long, and technical post (Bookworm may have over-simplified it in his)
Airfield QNH is measured by taking from a calibrated precission instrument the observed pressure at aerodrome level. This is then adjusted by the Met Observer to sea level pressure by applying a correction. The correction allows for deviation from the ICAO Standard Atmosphere (ISA) by taking into account prevailing temperature and any extremes of pressure.
This corrected value is given to ATC who will pass it to the pilot who should be able to set it without having to make a further correction. (The idea of using an old altimeter to obtain QNH is antiquated, it might still be used by some but IMHO they really should know better?)