When you are going to pick up your car, show necessary documents and you are set to go.
TIp: Be on time to pick up your car.
Rental car companies usually hold your booked car around 60 minutes after your reservation time during normal office hours.
When you will be late with your pickup or you need pickup time which is off the normal location opening hours then you have to request it and this comes with an additional fee.
In case your flight will be delayed, the car rental company will see it and most likely hold your car. There are exceptional case where the can cancel your booking.