For a quick glance here is a video - Creating non-billable appointment for a client
You might not be familiar with this feature as it a minor one but is very useful. From the Calendar View page, you can schedule a non-billable appt for a client.
To do so, double-click on the calendar to create the appt. Fill out the necessary details of Client name, auth, service type, provider name, location, schedule start and end date. Then check the "Do not Bill" option as shown below.
Save the information and you have successfully create a non-billable session for a client!