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Thanks for the question.
"Flushing (per GM)
You can use various methods and equipment to flush the cooling system. If special equipment is used (such as a back flusher) follow the manufacturer's instructions. However, always remove the thermostat before flushing the system."
I typically run a Prestone cooling system flush agent through the system. Lettting it run for a while with the flush agent in the system or however long the directions say. Then I open the drain on the bottom of the radiator to allow the system to drain, and at the same time, leave the car running with a hose in the radiator so freah water is constantly ran throught the system. After about 15 minutes, I turn the car off and allow the system to drain, close the drain plug the re-fill the system with a 50/50 mix of water and coolant. I find this method to work very well.