You walk out the door, cross the street (carefully, they have some nuts there, enter the garage and go to the National level.
Go to the aisle corresponding to your car class, select the one you want, go to the exit, hand them the car info you will find in the car, your license, coupons (if you have booked with them) I bring a copy of my reservation and give that to them, your credit card, they will check you out and you are on your way.
Very very easy. Let me know if I can help further.