Click on the time in the diary that you would like to book the appointment for.
Once you have selected the patient or create a new patient you will be taken to the appointment detail section
There are a few other pieces of information you can add when booking the appointment:
Notes - if you have any notes you wish to be made on the appointment
Recurrence - You can put in a recurring appointment should the patient wish to attend at the same time every week for example.
Waiting List - Select this if the patient would like to be placed on a waiting list and notified if an earlier appointment becomes available in the clinic.
Clicking on an appointment will open the 'Edit Appointment' window: