Here is a procedure I copied from another Forum:
1) Buy 4 gallons of Ford Premium Gold coolant at dealer (or Zerex G05)
2) Buy 15 gallons of distilled water at local grocery store.
3) Drop lower radiator hose until empty.
4) Reconnect hose and add distilled water to degas bottle until full.
5) Drive truck until temp guage goes to normal and thermostat opens.
6) Repeat steps #3 through #5 no less than 4 or 5 times (depends on distilled water amaount used) - I can't remember?
7) You now have 100% distilled water in your system.
8) Drop lower hose 1 last time, drain and reconnect.
9) Look in manual for your system capacity and for a 50/50% refill of coolant just add the coolant straight into your degas bottle up to the amount that equals a 50/50% concentration. Our refill capacity is 27.5 quarts.
10) Start motor and add distilled water to bring system level to the MIN mark on degas bottle.
11) Drive truck around to open thermostat and let cool 1 or twice to purge any air. May need to add more distilled water (don't add coolant)... but when done you have flushed your entire system and have a perfect 50/50% concentration of coolant and distilled water!!!!
The dealer only uses tap water and so does most other places... distilled is best in order to keep the minerals out of your system.