Once the growth medium has boiled long enough to sterilize it, take off the heat and pour right from the flask and into the Petri dishes. The hot liquid helps to sterilize them again, just in case. They need time to cool and solidify after they're poured. This is where the highest risk for infection is, so what I do is keep a steady updraft going right next to them so anything floating in the air doesn't have a chance to land. This was achieved by putting the plates on one side of my stove, turning the burners on on the opposite side of the stove, and also turning on the exhaust fan.