To make a check-in, simply change the reservation status to Checked-in. You can do that from the right click option:

Or inside the reservation form - set status and save the changes. 

When you make check-in, make sure that you add all the required guest details to the reservation. If some details are missing, the system will ask you to fill out the missing details first. They will be highlighted in red.

> See how to change reservation status

> Check how to set up required fields for creating a reservation or checking-in a guest