A new appointment can be booked by clicking on the + button (up, right) or on the corresponding free space in the calendar.
By doing so, a new appointment page will open where you can define your details for this appointment.
add a customer from your actual customer list or create a new customer by clicking on the + sign
select one or several services for this specific appointment
and even define which of your employee should execute this/those service/s
add custom service
and finally you can click on "Save" to save your appointment.
You can also click on "Send & Save" to save your appointment and also send it to your customer