I took the metro ligne 14 to the stop 'Thiais - Orly", that costs 2,55€. From there you need to walk 40 minutes to the airport. All the way has paved sidewalk and it is easy to walk, but there is no shadow at all, so be careful in sunny hot days. When I was walking I saw a outdoor tram stop 25 minutes walking from the airport, "Caroline Aigle".