plane install

Most downloaded aircraft from FlyAwaySimulation has an explanation of how to install if you download the Base Models.  Basically, you need just a few files;
Model folder will have a model configuration file in it specific to the aircraft.
Panel folder will have a panel configuration file & may have a few bmp & cab files in it (depending on the plane, some refer to default aircraft panels & do not need specific files).
Sound folder will have a sound configuration file & some sound files to go along with the mechanics of the plane.  Again, it may rely on the default aircraft & therefore not need any.
Texture folder will have some texture files specific to the aircraft.
You will need an Aircraft configuration file & an aircraft .AIR file.

These files are all copied into a folder named after the plane you chose, in the
<C:\Program Files (x86)\Steam\steamapps\common\FSX\SimObjects\Airplanes> folder
or if it's a helicopter, you can place it in the <... SimObjects\Rotorcraft> folder.
When you start the sim, the plane will appear in the program aircraft listing.