Drain coolant. Fill with DISTILLED water. Run engine with heater on HI until engine reaches operating temp. Let engine cool. Drain coolant.
Repeat 2-3x.
Engine flushed.
Fill with 1/2 the listed volume of coolant (100% antifreeze) in the manual (eg, if the manual says 7.5L of coolant capacity, fill with 3.75L of 100% coolant). Top off with distilled water (this gives you a 50:50 mix).