Download Cars (one-by-one) @ ASR Formula 1991 - Developer Ressource

Pull the file onto your CM, to install content

OR

Download Track @ Bremgarten 1950

The <INSTALLATION_FOLDER> usually is located in:
C:\Program(x86)\Steam\steamapps\assettocorsa\

You can find your Track-Directory in:
<INSTALLATION_FOLDER>\content\tracks\ <---

The track directory of the new track should reside here.