There are products out meant for radiator flushes. You pour some or all the fluid out, put this liquid flush product in the radiator, run for a certain amount of time.. Probably better to run long enough to get hot and run with the heat turned on. Some cooling systems have a coolant cut off valve so the heater core is shut off from getting hot coolant and then opened to flow hot coolant once the driver demands the heat on. This is why all coolant flushes suggest turning on the heat so you are sure you are getting your heater core also.
I will warn you that some people use radiator stop leak to stop a radiator or heater core leak and in turn it plugs up the radiator. You might discover a leak after using radiator flush cause it removes the 'stop leak'.
Anyway, after running the flush product through for a while you drain it out and i think you rinse out the system and then fill it up with your appropriate antifreeze 50/50 mix. I did remember hearing someone talking about a way to 'beat the book' (faster than quoted time) on a kia heater core i think. Heater cores can be costly in labor.