To drain it all out, make sure the engine is hot and remove the bottom radiator hose with the top off the coolant expansion bottle. Might want to wear gloves though to stop yourself needing skin grafts. Refill back up with your mixed coolant after re-attaching the hose, when the level is right in the expansion bottle, give the rad hoses a squeeze to help the air out. Leave the car ticking over until got to allow all the air to get out of the system then top up again because the level will drop