Physically:
Goto an auto parts store & look for a flush kit that connects to a garden hose, Prestone makes one
You would hook up the the hose/kit at the back of the motor where the coolant hose is.
Connect that car hose to the garden hose using the kit with the radiator cap as well as the mentioned bolt/plug at the back of the engine block off.
Turn on the water.
Run the Engine.
That is the most thorough way you can physically do it.
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Chemically:
Now there is also a chemical cleaner you can use that is (according to my friend) only available at the dealer. It is supposed to remove any built up crap in the system. As far as the way it was mentioned to me, it is much stronger than what you would get from an Advanced Auto Parts or other stores like that.