Click on Settings from the left-hand sidebar.
Click on Other integrations.
Click on Connect link in
You will be redirected to a page which will have instructions about "Connecting your Google account".
Click on Sign in with Google to connect your Google account.
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.