This is a yeoman generator, so you will need node.js and yeoman installed in your system. Node.js is easilly installed from here.
To install yeoman and this package, run
npm install -g yo generator-beamng-app
If everything goes well, you are good to go.
To launch the generator, run
yo beamng-app
After answering a few questions about your app, you will be informed about the files generated. Edit them to achieve the functionality you want and then copy the folder to your mods directory (usually %localappdata%\BeamNG.drive#.##\ui\modules\apps).
That's it! Your app should now be listed under the available apps in the game selector.