I have distilled water,50/50 pre mix 12qts coolant and some bottle of Prestone flush stuff.I think it's a 16oz bottle.Distilled water to flush is ok to mix with that flush additive.Right?
It sounds like you have the right stuff, Most places do not use distilled water though because it really is not that critical.
To be frank, flushing a cooling system really isn't easy without the right equipment or disposal means, sure you might save a few dollars and you will find that used coolant is one of the worse liquids to ever put on your skin.
In and ideal world, The only way to flush the cooling system is to start with it cold and finish it with the thermostat open, That way you flush the heater core and the radiator.
If your cooling system is operating normally, It will be full, So how can you add the flush chemical?
You have to remove a lot of existing coolant to get the one bottle in (kind of weird how the cooling system works).
I should stop here, It is easier and cheaper to have a reputable place do this for you, Coolant at 200+ degrees is no fun to deal with.