This was done on a 1982 300D, though it should be applicable for other MB models.
I would recommend that this be done every 25-30K miles.
You will need the following:
1 Power Steering filter
1 Power Steering reservoir lid gasket (replace it if you don't know when it was last replaced)
1 gallon water bottle (to catch fluid)
3-4 quarts of cheap Power Steering Fluid of choice to flush with
1-2 quarts of Power Steering Fluid of your choice to replenish the system
1 Phillips head or flathead screwdriver depending on the head on the clamp screw
1 Adjustable wrench
1 new return hose clamp (optional)
(Note: You may want an assistant to help, but you can do this yourself.)
1. Open the hood and find the power steering fluid reservoir.
2. Loosen and remove the nut on top of the reservior lid with your adjustable wrench and remove lid.
3. Place the lid and nut aside, move any obstacles in the way and place your 1 gallon water bottle next to the pump.
4. Unscrew clamp on return hose to the pump with screwdriver, remove hose and put it in the bottle, you'll probably spill fluid in the process.
5. Start the car, and have either the assistant or you turn the wheel lock to lock. Pour 3-4 quarts of the cheap fluid in the pump while running and continue turning the wheel. It'll run through quick. Be careful of fluid overflowing from the catch bottle. Stop the engine when all fluid has been run through for flushing.
6. Carefully remove metal collar and spring holding filter down. Remove filter from bottom of reservoir. Place new filter in, "holes side up". Place spring back and push the metal collar down enough to hold enough tension to keep the filter in place.
7. Reattach return hose, make sure clamp is tight enough, or replace with a new one if you feel you want to do so.
8. Fill reservoir with fluid of choice up to "full" line. Start the car and turn the wheel from lock to lock. The fluid level will fall, replenish little by little until the level reaches full and stabilizes.
9. Remove old gasket from lid and replace with new gasket, return lid to reservoir and tighten up the nut. You're done.