Remove the plugs from the top of the thermostat housing and the top of the radiator, take the cap off the expansion tank. Top the system up via the thermostat housing until coolant comes out of the radiator - check it flows free from bubbles. Once done refit the plug to the top of the radiator and top up at the thermostat housing until you get water out of the top of the thermostat housing then refit the plug to this.
If it continues to get air in the system, was the head skimmed? Also is the correct thermostat fitted with a bypass disc on the bottom, also the thermostat cannot be removed to run the engine as the water will not enter the radiator and circulate through the block only.
Hope this helps :lol: