By default, three messages are created in your Auto Messages tab: a phone call, an SMS and an email, all set to go out 1 day before the appointment (not active until you turn Auto Messages on).
To create a new auto message, hover over one of the green plus icons, to easily create a message type of your choice with a specified timing “purpose” (e.g. Initial/Booking, Reminder or Followup), or you can click the blue “+ Message” button, shown below.
Clicking any of these options will bring you to the Message Editor to customize the language and timing of that new message.
To move any of these messages to a different rule, simply click and drag that message to the desired Rule block on the left: