Couldn't be easier. You park, and depending on how far from the airtrain station you've parked, you can take a shuttle bus to the escalators/elevators that get you up to the airtrain entrance level. Or you can just walk from your car to airtrain entrance if it's not far or you have a manageable amount of luggage. Simply follow the signs once you've parked... There's one airtrain station that's convenient for long term parking lot "A" and "B" (the "Lefferts Station - Stop B")- there's another station that's convenient for long term parking "C" (the "Howard Beach Station - Stop A").
Couldn't be easier. You