First off, why are you doing this?
But to answer your questions:
Its obviously better to drain it when the car is not hot, so you dont burn yourself. Once you fill with new coolant you have to bleed the system of air. With the radiator cap off and engine running, top off the radiator. You can turn the heater on, its not that integral of a step. It would speed the proceedure to give your throttle cable a tug once every few seconds. You want to have the coolant in the radiator up to a boil. When it starts boiling out to add more to keep it topped off. If you have a air-bleed screw, open it in the middle of this proceedure. You want a continuous stream of water to come from the bleed valve, if its squirting, you still have air in there. Proceedure should take 7-10 mins.