You would need to go to your "Calendar" page > select on the "Create" button (left hand side of the calendar page):

Select on the "New Appointment" option:

Which should allow you to first select on the service , choose on the assigned employee & the duration:

After filling out the information, choose the date of the appointment from the calendar:
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From the chosen date, you will be able select on the available employee and time of the appointment (the green time slots would show the available times, while greyed out ones mean that the employee isn't available at the chosen date/ time):

After a time is selected against the employee's availability > you can now either search on the existing customer:
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Or "add a new customer":

Fill in the necessary information of the appointment:
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Scroll down for other optional settings of the appointment, such as: enable reminder by text/SMS, repeating appointment, payment (if needed), & you can also suppress confirmation messages: