I recently changed the coolant in my '88 260e which is basically the same motor as your 300e. First I drained the coolant from the two drain valves mentioned in the previous posts. Then I took a garden hose with a spray nozzle and flushed the remaining coolant out through the reservoir and radiator cap, then removed the hose from the monovalve and flushed through that. It seemed to get all the old coolant out. I poured about 1/2 gallon of distilled water through the radiator and buttoned everything up.
When refilling, make sure to use equal parts of water (I use distilled) and coolant. Too much coolant and you lose cooling efficiency. Refill slowly and keep squeezing the upper radiator hose to burp all the air out. There is also a small bleeder screw on top of the head but I didn't find it very helpful.
I didn't find the process any more difficult than other cars except for the monovalve setup which causes some difficulty in flushing the heater core.