Click on Settings from the left-hand sidebar.
Click on Other Integrations.
Click on Google Calendar among the list and click Connect Google Calendar.
A page will be shown with instructions about connecting your Google account.
Click on Sign in with Google to connect your Google account.
Follow the process from Google's side. Don't forget to check the checkboxes on the Google consent page.
You have successfully connected to your Google Calendar account.
If you want to select multiple calendar accounts or add a new one, please check out this article: https://help.neetocal.com/articles/multiple-calendars.