>>341909727
Unpack the files, go to the Assetto install folder, drop the car folders in content\cars and the track folder in content\tracks. It's also a good idea to test in advance by connecting to the server to make sure everything is working.
You can also drag and drop onto the Content Manager window to install if you're using that, but it occasionally fails, so I just do it manually. Content Manager is worth running either way though.
https://assettocorsa.club/content-manager.html