Point the iPhone's camera down at the ground and walk around the area to "Paint" the ground where the vehicle will be placed on the location and touch the "Set" button.
The default vehicle (Crossover) in the default color (silver) will be dropped on the "painted" surface. Walk back to the desired camera position and start
to choose your options.
When the apps starts acquiring the surface, the "infinite plane" button is set to "off" by default. This limits the area where the vehicle can be placed to just the section of the surface that you "painted".
You can extend the surface to an "infinite" surface area by changing the "infinite plane" button to "on".
The app might have trouble finding the surface at night or if the location is dark.
*The app will only recognize a surface that is less than 3 meters from the iPhone. So, if you want to shoot a "Top Down" angle of the location, aquire the surface by walking around and scanning where you want to place the vehicle and set the surface.
Then, with the app open and the vehicle set on the surface, walk around to your top down perspective and set up the shot.