A proper flushing of the cooling system involves the engine block as well as the radiator. In order to reach the block plug, the car will have to be up on a lift or on jackstands. If it's been some time since the last flush, say over 3 years, it might be a good idea to run some Prestone radiator flush throught the system as a preliminary.
Pay attention to what type of coolant you use, make sure it is compatible with aluminum systems, and avoild silicates, borates, and phosphates, plus some other stuff. I use Prestone Pre-mix Dex-Cool, good for 5 years/50,000 miles, although I don't intend to keep it in that long.
Oh, wear safety glasses, the old stuff can splash around everywhere...