When I changed the coolant on my partner's 2011 diesel, I drove the car around until it was up to working temperature, carefully released the header tank cap, then used the drain tap at the bottom of the radiator to drain off as much of the old coolant as I could into a bowl. I then closed the tap and refilled with demineralised water which I heated up on the stove in a big stock pot. Then I drove it for a few miles to make sure the fresh water was distributed evenly around the whole water circuit. I then repeated the exercise a few times until the drained water ran out clear. The first 2 or 3 flushes produced murky brown water but eventually it cleared up, so I was satisfied that most of the sediment was flushed out, and that the concentration of existing coolant would be very low.
On the final fill, I first added enough concentrated coolant to give a 40% ratio, then topped off with more demineralised water. The tap water in my area is very hard which I why I didn't want to use it, and one of the local supermarkets sells demineralised water at a really low price. In total, I used about 20 litres of water.
I found the header tank was pretty dirty and it's hard to tell the level of the coolant through the plastic. in hindsight, I would have tried cleaning up the header tank first.
