To perform an automatic transmission flush, use the vehicle's transmission pump to simultaneously force the old transmission fluid out through the cooling lines, while drawing new fluid in through the dipstick tube. Have a second person assist you when performing this task.
Gather a jack and jack stands, a 5-gallon bucket, a long funnel that fits into the dipstick tube, 12 quarts of transmission fluid and a wrench or screwdriver, depending on your vehicle's hose clamps. Run the vehicle's engine until it reaches the normal operating temperature, and shut it off. Jack up the front end of the vehicle, and place it on jack stands.
Remove the hose clamps from the transmission cooling lines where they enter the radiator and slide, or twist the hoses loose. Place the hose ends into the 5-gallon bucket. Remove the transmission dipstick, and insert the funnel into the dipstick tube. Restart the vehicle, and add new fluid to the funnel as the old fluid is forced out. Don't let the funnel run dry. When 9 quarts have been added, shut off the engine.
Lower the vehicle, start the engine, and run it until the normal operating temperature is reached. While depressing the brake pedal, move the shift lever through all gears and then back into Park. Check the dipstick. If it indicates more fluid is needed, repeat the process until the correct level is reached. Don't overfill the transmission fluid. If there is too much fluid, remove some through the drain plug on the bottom of the transmission.