The most important thing, of course, is to be very focused in setting the fluid level when you are are finished with the task. You will need a proper dipstick TOOL to do this (pictured below).

Also, unless you have a very early W208 (first model year, really), you will not find a drain plug on the torque converter (disappeared in MY 99). In most cases if you want to effect a complete change after dropping the pan and replacing the filter, you will have to then use the cooler lines to flush the rest of the system.

Good luck.