Pull the block drain bolt, leave lower hose off the radiator, pull the t.stat. Stuff a water hose where the t.stat. would normally reside, and turn it on full. Let it run until it runs clear out of the drain bolt hole and water pump. Turn the water off, pull the hose out of the t.stat's place of residence, and stick it in the radiator where the upper hose goes. Turn the water on full again, and let run until it comes out of the bottom clean. Now, at the most, you have water and whatever built-up crud that was already in the block, in there. If there is a lot of brown crap coming out of the block, or the drain plug hole is so plugged up it won't drain, you should still run the hose in there until you get clean water out, put it all back together, and drop in a bottle of Prestone coolant system flush, and fill with straight water. Read the directions for how long to leave it in there. After the flush has been in there the correct amount of running time, Flush again with the hose, same as before. Let all the water drain out, button it all back up, and fill with 1/2 water, 1/2 coolant.
There, now you have an idea of how to flush the coolant system. Prestone also makes a coolant that will mix with either red or green. Use it, and don't worry about it. You can buy it premixed as well.
