Go to the auto parts store, or I believe even the discount stores, and get a Prestone flush and Fill kit,(4.00).
You install a garden hose fitting in your heater hose,which remains on the car, and is capped up,(supplied in the kit).
Then you attach a garden hose/backflow preventer (all in the kit).attach your garden hose, and start the car, and run the hose/car until the deflector,(in the kit) runs clear water.You will literally see that clean water runs out.
You do all this with the radiator drain valve open, let all the water run out, close the radiator drain and add coolant until the garden hose connection shows the coolant running out.Put the cap back, on the fitting,and put on the radiator cap.
You have a clean radiator, block, heater core,and a 50% dilution of coolant.
I usually run a good flush treatment, for a day or two before I do the flush.This loosens all the crud, and cleans the surfaces inside.
If you want to be more Eco friendly,before you flush,drain the old coolant/flush into a tub,(I use one of those black plastic oil drain pans), and pour it into your toilet.The sewage treatment plants are set up to remove the ethelyne glycol, and this is a perfectly acceptable disposal method.Some auto parts stores accept used coolant too.
Not saying you need to use Prestone, when you finish the flush,you only have clean water, so use whatever you wish for the anti freeze.
I prefer to use Benz coolant,from the dealer.