Ok,
I have started now.
I bought a 10mm hex key for the throttle body connection and 6 litres of coolant and 5 litres of deionized water.
Then I opened the radiator valve and poured the coolant away, filled up the expansion tank and bleed the system. Repeat of this 3 times, then I started to flush the car directly, so left the radiator valve open, run the car 5 minutes at time with constant water flow to expansion tank.
Eventually water was clear, when I started it was dark blue, black even.
Then I added the deionized water, bleed the system, emptied the radioator and poured coolant trough throttle body valve.
Now it is quite good, but since I have only poured about 2½ litres of coolant in the system, there is too much water, so I will wait the car to cool down, open the radiator valve, empy the radiator, fill up trough throttle body valve and then bleed the car again.
Takes forever but I think the result will be good :)