To integrate a calendar, first navigate to Ctrl-Ctre, then go to Configuration and select Calendar Integration. Edit the user by adding their Google or Outlook email address, then click Save.
Next, in Calendar, click the cog icon and select Calendar Integration Settings. From there, sync the calendar, choose the appropriate calendar to sync (such as Lettings or Sales), and enable either Outlook or Google Calendar Events. Finally, enter the account password to complete the connection.
Further Reading: Creating an Entry in CRM Calendar and Configuring CRM Calendar