This is how you do it yourself. Only other way is with a flush machine @ the dealer or indie shop, which many have criticized for it's habit of dislodging chunks and clogging lines and valve bodies.. This requires 12-14 quarts of fluid in order to completely flush out the torque converter. Draining/flushing is done through the cooling return line from the tranny @ the radiator connector.

In a nut shell....

In order to perform a correct flush (Actual MB Procedure):

Remove the fill tube cap.
Remove and replace the pan filter
Fill pan with 5 liters of fluid.
Remove banjo bolt on fluid return (Pax side of transmission, route line to bucket)
Start car, count 3 liters of fluid, shut car off.
Add 3 liters of fluid.
Start car, count 3 liters of fluid, shut car off.
Add 3 liters of fluid.
Start car, count 3 liters of fluid, shut car off.
Re-install Banjo bolt.
Check sump level using dipstick tool (Factory tool), add as required with sump at 80C.

Thats a 100% fluid flush and should be accomplished every 30,000 miles on the 5 speed transmissions.

Anything less and you are just wasting fluid or delaying a worn out transmission valve controller.