I start the engine, stick a hose pipe in the top of the expansion tank and then remove the bleed screw when flushing mine out. I then leave it going for about 15 mins. When I'm happy it's been flushed I take away the hose and let the water level drop almost to the bottom of the expansion tank before topping up with antifreeze. I repeat this several times until all the antifreeze is in the system. When I'm happy the coolant is at the correct level I replace the bleed screw.