Requirements:
Script Hook V
ScriptHookVDotNet
.NET Framework 4.8 (or newer)

Installation: just drag the file into the scripts folder, located in the game's root directory. 
If the folder does not exist, create it (making sure you have met all the requirements above)

You can also add your own aircraft. To do this, save it via the Menyoo spawner, then go to 
Grand Theft Auto V\menyooStuff\Vehiclee and find the file named after your aircraft. 
Open it with Notepad++ (or any other text editor) - the first line will contain ModelHash. 
Copy it, open the .cs script (also opens with notepad++), and paste it in following the same pattern as the other hashes.