From a post by Vilan in another thread:
"A proper flush isn't hard to do. I just did my first one a few weeks ago, no big trouble. Took me about an hour, but most of it is spent waiting for the coolant to either heat up or cool down between flushes.
Basically, after you dump your old coolant out of the radiator, fill it with water (distilled is best, but tap water is OK if you don't have hard water) and turn on your engine for 5-10 minutes to get the flush flowing. Also turn your car heater on maximum to let the flush get into the heater core. Then let it cool off for another 5-10 minutes and dump out the water. If it's clear, you're done. If not, do it again until you've gotten all the old coolant out. I did 3 flushes and it was still a little green from my old coolant, you may need even more. If your rad is really bad you might want to buy some flush products and use that instead of water, your call.
If your car has engine block coolant drain plugs and you've opened those to start with, probably one flush is enough since you won't need to worry about having coolant trapped in the engine. If you don't have engine plugs (or can't find them ) then also remember that your engine will be full of water after your flushes, which will dilute the coolant you add afterwards. Pay attention to how much comes out of the radiator and compare it to the total capacity (should be in the owner's manual) to see how much is in the engine, and adjust how much coolant you add accordingly to get the coolant:water ratio you want."
If you don't use distilled water in each cycle of the flush, then when you're done flushing and fill the radiator with antifreeze and water, all the liquid in the engine block will be tap water, which is not preferred. If you use distilled water for each cycle of the flush, after the last cycle the liquid in the engine block will be distilled water. Distilled water is cheap; use it for each flush cycle.