Log in to a Google Account that has access to the calendars you want to display, go to the API & Service Credentials page and create a new project.
Click the Create credentials button, select OAuth client ID and configure a Consent Screen first (enter a product name of your choice and leave everything else blank). Then chose Other as the application type using any name you like.
Download the credentials using the Download JSON button and put it in a sufficiently secure location (it does contain your client secret), e.g. ~/.gcd-auth/credentials.json.