You are able to send SMS appointment reminders manually as well as automatic reminders. To find out how to send automatic reminders, click here.
To send a manual reminder click on the appointment. This will bring up the appointment details screen as shown in the image below.
You should then click on 'Communication' which will bring up a list of the types of communication you can send. Click on 'SMS' and then choose 'Appointment Reminder.'
You can find out how to set up SMS templates by clicking here.
This will then bring up the screen below containing your appointment reminder text. You should then click 'Send.'