There are 4 ways to check members into an appointment.
Method 1 - Dashboard - My Day panel
from the Dashboard, if the appointment is assigned to you, you should see it in the 'My Day' panel.
Use the 3 dots context menu to check them in.
Method 2 - Dashboard - Check-ins panel
from the Dashboard, use the 'Check-ins' panel and select 'Scheduled Appointments' from the dropdown where you would typically select the class time.
You will see all scheduled appointments for the day below. Check the box next to members name to check them in.
Method 3 - Appointments page
Go to 'Schedule An Appointments' page
Find the appointment on the schedule. Click it.
Click the 'Check-In' button on the Appointment Detail card.

Method 4 - Check-in App
Use the self service tablet Check-in app to check them into their appointment.
The member types in their name
Their appointment will show up in the class list.
Member taps the 'Check-in' button.