As first time to return car here at this airport and rent from other location. As smaller airport, where is car return location, which car park building? Or it had car return sign to follow easily.

Underground

Car return at John Wayne is right at the airport, entrance will be on left side of entry road, and it is in the lower level of the garage, right across the street from the airport. Signage is pretty good, but pay close attention since it may be easy to miss, as the entrance is right after a sharp curve. Garage is walking distance to the terminal, no shuttle necessary to get to your flight :)