Park in one of the arrivals bays and head to reception to check in. You should have a copy of your booking confirmation with you. You'll need to leave your keys with reception, as your car will be parked for you. You'll be given a ticket, which you should keep safe as you'll need it when you return. Then just wait for the shuttle bus to the airport in the waiting area. You'll need to show your ticket to the driver.
If you have sports equipment or lots of luggage, you may need to wait for a bus with more space during busy times, so make sure you leave yourself plenty of time to get to the airport.