I remove the bottom hose to the radiator, drain the coolant, flush it out with fresh water, refit the hose and refill the cooling system from the top of the rad.
There should be a drain plug in the block to drain the coolant but usually they're a pig to get to and usually they are rusted in so ive never known one to be used. You can also remove a heater matrix hose and flush that out aswell.
Jay