From the Bentley Service Manual:
1. Place vehicle on lift.
-You can jack the car up if you like. BUT IT MUST BE LEVEL.
2. Warm up drivetrain by running engine until ATF reaches specified temperature (30-55C) (86-131F). Apply brake and, while idling, shift through all gears several times. Turn engine off.
-Be careful while having the engine running while in the air. Make sure the car is completely stable on whatever it is lifted on. If you do not have the proper temperature devices, you can use rough estimating by using touch. The trans pan should be warm the the touch, but not hot.
3. Remove ATF fill plug. Place transmission in neutral and restart engine.
-Make sure to pull the FILL PLUG. It is located on the side of the oil pan, about half way up. DO NOT PULL THE BOTTOM PLUG.
4. Fluid should drip from the hole. If not, add fluid until it drips from hole.
5. Re-install plug with new seal ring.