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^^what he said^^Run a garden hose or a large funnel into one of the radiator hoses (with the thermostat out) and pump craploads of water through. Then do a reverse flush.
Then fill up with a bottle of coolant flush concentrate and tap water, close the system (still without thermostat) and run the car for half an hour. Turn off, wait to cool, flush the entire system again as you did before.
Then if it's particularly bad, fill with coolant flush and do it all again.