If you have pre-booked parking, you will pay at the time of booking.
If you have not pre-booked, you will need to either tap your credit card upon entry or take a ticket. If you tap your credit card, you will need to tap the same credit card at exit. You will be charged accordingly, and the boom gate will open.
If you take a ticket, you can pay at the pay station located in the terminal when you return and then insert your ticket at the exit gate. Alternatively, insert your ticket upon exiting and pay at the gate.