You may now add an appointment within the "Customers" page; under the "Customers" page search for the customer's name, email address or phone number to access their account:
Then it will lead you to a page where you can see the customer details and some appointment information. Click on the plus sign (+) to add an appointment:
It will lead you to the Calendar page where you can add the appointment from there, here are the steps on how to add a new appointment:
Adding new appointment (using a Flexible schedule): https://help.flexbooker.com/en/articles/90260-making-a-new-appointment
Adding a new booking (using a fixed time schedule):