1. Create a project in Google Cloud Platform with an appropriate name
  2. Go to credentials in APIs & Services
  3. Create new credentials for an OAuth Client
  4. Add the domain name to the javascript origins and the redirect link. (Note: see here for issues with HTTPS in production)
  5. Add a secret and copy that somewhere safe
  6. Edit the OAuth Consent screen with only non-sensitive scopes (otherwise you will have to go through a verification process)

See Also