Should be very easy. Drop it in Aircraft/Extra AIrcraft and it will show up in your aircraft directory within X-Plane. You have to buy and download it from store.x-plane.org/vFlyteAir_c_175.html It will download as a zip file, unzip it and drag the Cessna 150 folder into the Extra Aircraft folder. VFlyteAir does a great job with those retro types.
