Click on the appointment that you want to generate a message for. Click on the SMS Options (2Hippo) button, and 'SMS/Email [Patient Name]':
Send SMS Dialog
The Send SMS Dialog contains various options allowing you to configure a specific message to a patient.
Confirmation & Expiry
Elect whether or not you want confirmation, (i.e., you want the patient to reply 'Ok' and automatically confirm their appointment).
You can also set the expiry here, (this only really affects the appointment if you have set the 'Cancel Expired' option).
From here you can set up the message format. You can either type directly in to the message box, and/or load a message format. The number of characters is also displayed, there is a maximum of 140 for SMSes.
When finished, press 'Ok' in the Send SMS Dialog, and press 'Yes' in the Send Message screen.