I do what red89notch does AFTER I warm the engine to operating temp. Then I disconnect the top transmission line at the radiator and screw in a fitting connected to a drain hose, (OR SLIDE THE HOSE ONTO THE TRANSMISSION LINE DEPENDING ON WHETHER YOUR TOP LINE IS PRESSURE OR SUCTION), into a five gallon bucket, (I added one quart marks with a magic marker on the side of the bucket up to 12 quart mark). Plug the open line to the transmission, (OR THE RADIATOR), to keep the dirt out.
Open the oil and set it where it won't get knocked over.
The next part works best if you have a helper in the vehicle.
Start the engine and as the ATF flows into the bucket - add a quart, at the filler tube, as a quart flows out. If it flows out faster than you can pour it in, (some of the older cars would pump slow enough that you could keep up), have your helper stop the engine if you get more than two quarts behind. When you catch up - have him/her start the engine and continue the exchange. When you have run the required amount out - shut the engine off when it is about a quart short. Connect the transmission line back up to the radiator and top off the transmission to the full mark.
Have done this on a number of vehicles with no problem. The only hitch is: on some vehicles the top tranny line is pressure and some are suction?
Believe it or not!