Fill until it is full (the radiator), and let it sit for a bit, move/shake the machine to get the air bubbles to rise up. Refill again (the air will bubble up to the top). Keep doing this until all the air is out of the system.
I run 50/50 during the winter, and a 75/25 with water wetter during the summer. The only reason I put any antifreeze in during the summer is to lubricate the water pump... just plain water will cause corrosion.