Drain the radiator and pull the block plugs and drain those too. The block drain plugs can be a real pain so what I usually do is: drain the radiator, remove the radiator hoses and heater core hoses along with the thermostat and put your garden hose in the engine and let it run for a while (the garden hose). If your persnickety about water, like myself sometimes, pour in distilled water and flush with that. I use distilled water for my 50/50 mix and I use the green antifreeze. Be careful about flushing the heater core with the hose, it can cause a leak.
