Click on Settings from the left-hand sidebar.
Click on Other integrations.
Find Google Calendar among the list and click on Connect
A page will be shown having instructions about "Connecting your Google account".
Click on the big blue Sign in with Google to connect your Google account.
Follow the process in Google side. Don't forget to tick the checkboxes in the Google consent page
Note: Sometimes a warning will be shown before the consent page. If so, please see the video below to understand what this warning is about, you can safely skip it by clicking "Advanced" and pressing "Continue"You will now be able to see a list of calendars in your Google account, and also you can set your calendar preferences accordingly.
If you want to connect another calendar account, click on the Add a calendar account link and follow the same steps as before.