If you really want to FLUSH the system, and not just drain it and re-add coolant, buy the flush kit at your auto parts store for around $10. It hooks up to your heater hose (the smaller hoses running to the throttle body), and has a regular water hose fitting on it. drain your radiator, and leave the cap off. Then turn the hose on while connected to the fitting that tees into the heater hose, turn the thermostat to hot, and start the car. Give it a minute or two to get all the water circulated thru the system, until it runs clear from the radiator cap. Turn off the car, and drain the coolant system again. Then cap off the flush kit, and start adding water to the radiator (distilled preferrably, as it doesn't contain minerals). Fill it up until it's topped off, and start the car. The water will disappear pretty quickly, so get under the hood and keep adding water. After one gallon, add a bottle of water wetter, and then add the rest of the distilled water until the system isn't burping bubbles anymore.
The easiest way to do this of course is like Dre mentioned. Just drain the system, and add fresh coolant while bleeding the air out.