if you want to do it really well then after you've rinsed the crap out fill it with water and add a chemical cleaner which will remove rust and scale and all sorts of ****e that a rinse out wont touch.
Wynns do a good one thats available just about anywhere
it will tell you how on the back of the can
i did it to my 97 300tdi defender and heater works a treat once the engine warms a bit lol