If you've booked Emerald Aisle, then you just head out to the garage and follow the signs to National. Then look for the aisle that says "Emerald Aisle". Pick whatever car you want on that aisle. The keys will be in the car.
Then you drive to the exit booth. They will want your credit card and your license. This is when you tell them you have another driver and they will want their license too I think. This is also when you'd give them any coupon you may have.
That's it.