Remove all water from the block

16 quart capacity
Drain 8
Refill with two gallons of non premixed.

Done, simple quick and easy.
If you even bother pulling the thermostat drill a 1/8 hole in the shoulder of it and put it back in.
Not only will it burp the air out of the block right away when filling it back up, but will do it that easy forever after. (with that specific thermostat in it)
I'll never put one in without drilling it to let the air out before the stat opens.