Set the ACC to defrost and set the thermostat to max heat. Turn off the engine, allow to cool. Remove the radiator cap. Loosen the drain plug at the bottom of the radiator. I have a W126 so a picture of my radiator drain might not apply. Open the engine block drain plug. You only have to loosen it, not remove it.
The block drain plug is shown in the picture. It's the stub visible between the #4 and #5 exhaust runners. It's very easy to get to the block drain plug from below but you might have to remove a splash shield. I apologize for the picture quality. I had to get it under the attachment size limit.
In most places, you shouldn't send coolant down storm drains or street sewers. It's usually okay to send it down the sink drain or toilet that flows to a processing plant.