1. Create or open the appointment or meeting that you want to make private.
2. Select the Private check box in the lower right corner.

Important  You should not rely on the Private feature to prevent all access by other people to the details of an appointment, contact, or task. To make sure that other people cannot read the item you marked as private, do not grant them Read permission to your Calendar, Contacts, or Tasks folder. A person who is granted Read permission to access your folders could use programmatic methods or other e-mail applications to view details in a private item. Use the Private feature only when you share folders with people you trust.

A better way to keep individual appointments private is to create them on a separate additional Calendar. Additional calendars cannot be shared.