Radiator flushing procedure for a 2003 GMC Envoy; and where can I locate the drain plug?

The check engine light has beein on for a couple of weeks on my 2004 GMC Envoy. I went to Autozone to let them hook up their tester and found out the it was slightly low on antifreeze. I added about 1 cup of regular Prestone green antifreeze, and now everything is good again. I've since been told that GM vehicles require Dexcool instead of regular green Prestone, so now I'm thinking I should flush out the radiator and do it right. What is the procedure for flushing the radiator? And where can I locate the drain plug on the vehicle?