Dunehopper has it, although with the WRX you pour the coolant in the tank near the turbo.
Leave the cap off and just keep watching and waiting for no more(or very very few) bubbles to pop up. Be patient
And when you do pour the coolant in, try to pour it slowly as not to create alot of unwanted bubbles.
Oh, and make sure your heater is in the HOT position so the heater core gets filled as well at the same time.