Go in Settings > Google Calendar, provide the Client ID, Client Secret and Application Name, then Save.
You should now see a Connect your Google Account button.
Click the button - if you are not logged into your Google Account, you will be asked to login. A consent screen will inform you that your Google Account is going to grant the authorization to the plugin in order to have access to your Google Calendars.
Continue until you are redirected back to the plugin page.
During the consent process Google might warn you that the App isn't verified. Just skip this advice by clicking on Advanced (at the lower-right corner) and continue.
At this point you are ready to activate any feature of the 3-way sync.