Obviously you need to make sure the coolant level is high enough, and the engine up to full temperature so the thermostat is open, then turn the heater lever to full hot and get someone to sit it the car with their foot on the gas to hold the revs up then undo the heater bleed valve, let coolant bleed out and then close it before any air is sucked back in when the revs drop. Nothing came out of mine without increasing the engines speed because the water pumps pressure is too little at idle, and my system doesn't hold pressure anyway.
