No idea if this is the official way of doing it, but its what I do...
Remove the bottom hose from the radiator. and let it all piss out over the floor. Don't do this on the driveway, do it on the road! lol.
Flush through radiator with clean water via hosepipe
Remove heater pipes & Flush through the heater (remove pipes at the bulkhead) with clean water
Put all pipework back together.
Fill the system up with Type D coolant from renault (its proper cheap compared to stuff from halfords etc)
run it up to temperature and bleed for air leaks via the bleed screw.
Let it cool down completely and top up as needed.