Important: for league drivers uninstall the previous version first with the 'Uninstall RDMCS10 Skinpack v1.0' located in the main folder.
Extract and run the installer file as admin.
Point to your Steamapps\common\Automobilista install folder.
To find the cars in game, look for the league logo on the main menu on page 2.
Uninstaller included as well in main AMS folder.