You can do this in one of two ways. The reservations page or Point of Sale. POS is fastest if you're goal is to take payment quickly.
To use the Reservations page: Click Appointments > Reservations
From this page you can take payment using the dropdown menus to the right.
From POS, you can tally up all outstanding appointments by adding them to one cart. Just search for the customer > select Appointments and view their outstanding reservations > Add them to the cart > choose payment method.