Disconnect the lower radiator hose from the radiator.
Flush the cooling system as follows (will take 10 minutes and a couple of gallons of water but will save you $50 in mechanic's fees): resecure the radiator hose. Fill the radiator with water. Turn on the engine and run the car for a couple of minutes until the car is warm. Turn off the engine, remove the lower radiator hose. Repeat this process until only water comes out of the radiator (usually 2-3 cycles are needed).
I've helped friends flush their radiators and it took hours because they never cleaned their radiators and we had to get all the rust (brown color) out. If you have any concerns about the quality of your coolant, add Prestone Superflush.
I strongly recommend Dex-Cool or Prestone 5/150 (orange antifreeze/coolant). It's only a couple of dollars more than the green stuff. It cools the engine better (as evidenced by the temperature gauge being lower on my 240s after I use a 50/50 mixture of coolant and water) and lasts for 5 years.