If you move the aperture ring on your FD lens from the "A" setting to a specific f/stop, your AE-1P will be in manual exposure mode. The meter should continue to display the aperture it would set in shutter priority AE mode, but your exposure will be at the actual f/stop set on the lens.
When using an FD lens, the AE-1P will not meter properly when you stop the lens down. In this case, the stop down lever should only be used to check depth of field.
However, with non-FD lenses (e.g., Canon Tilt-Shift, FL or R mount lenses, or preset lenses), the AE-1P will provide a form of match-needle metering when the stop down lever is engaged (i.e., you must meter with the lens stopped down). On preset lenses, remember to close the aperture to the desired f/stop. Using the AE-1P's meter in this mode, set a shutter speed and aperture combination and press the shutter button halfway. If the viewfinder indicates "5.6", you have the correct exposure. If it reads higher or lower than "5.6", change the shutter speed and/or aperture until the viewfinder reads "5.6." Don't take the "5.6" literally. In stopped-down metering mode, it only indicates correct exposure at whatever aperture you set. A readout other than "5.6" simply indicates over or under exposure.