On Canon FL cameras with TTL metering, readings are metered with the lens stopped down. When the stop down lever on the camera is pushed toward the lens, (a) the lens' diaphragm closes and (b) the meter activates. The A-M switch on an FL lens is used to preview depth of field or to stop down the lens when it's not mounted directly to the camera (i.e., when using a bellows, extension tube or lens reversing ring).
Stopped down metering on F series, FD mount cameras (i.e., original F-1, EF, FTb, FTbN, TLb, TX) is basically the same as on FL bodies. Turn the meter on and push the camera's stop down lever toward the lens. On A and T series cameras, FL lenses will only meter in the stopped down automatic exposure mode. Otherwise, FL lenses should be used in manual mode. This photo.net thread may be of interest:
http://photo.net/canon-fd-camera-forum/00Qdxg
Be aware that the FL 58mm f/1.2 lens is one of a handful of FL lenses that Canon says should not be used on most of the A and T series cameras. Some manuals state that this lens will not meter correctly and/or may interfere with the camera's aperture signal levers. On the other hand, some users have reported no problems using this lens on their cameras:
http://photo.net/canon-fd-camera-forum/00QX8Z
My choice of camera for FL lenses would be an original F-1, FTb or FTbN. FL bodies with TTL metering (Pellix, FT, TL) are fairly plentiful and cheap, but are more likely to need service due to their age.