to get everything out - make sure the engine is dead cold (not run for the last 5 or 6 hours)
remove the radiator cap, thermostat, both heater hoses and the radiator drain tap. Get a garden hose pipe with very low pressure water flow.
push it in the heater pipe, let it flow for 5 minutes, do the same on the engine, it will drain out the head, block and radiator completely.
you then use a blower or compressed air at low pressure to push the trapped water out of the system. I have made a self made system that pushes RO water through the system to flush out the tap water I used before, the system is just a 15 litre plastic bucket with a hole in its bottom and a pipe nipple tightened in it and a pipe attached to it with a little valve in line - I put the bucket on an elevated surface and the water just pushes itself through the system and out.
pack it up and fill with coolant - (make sure to burp it of any air). - The above self made bucket thing is good, as I just now fill with exactly half volume of the system with concentrated coolant and start the engine.
ideally a citric acid flush is best to do - (taatri from karyana shop) - it is the absolute best radiator cleaner you will ever buy - forget any brand name flush from liqui moly etc, they are all trash. The 10% citric acid flush is recommended by Mercedes Benz and Rolls Royce to descale cooling systems.
