Once check-in has been enabled, the Upcoming Bookings list on your Home tab will have a button next to each upcoming appointment "Check In". Clicking that button will check-in a customer and add them to the "Checked In Bookings" list at the top right of the screen.
Any customers who self-check-in will also appear on that list.
Once a customer has been admitted for their appointment, your staff can mark them as admitted. There are also options to undo a check-in or admission if a mistake has been made.