In order to register a Check-in or a Check-out, go to Planning and then Calendar in your Amenitiz administrator area left sidebar.
Once here, you will find all your bookings on a calendar form. Click on the booking you wish to take action on.
Scroll to the room details in the summary section, you will find the buttons Register Check-in and Register Check-out. Click on it to register the information:
If you wish to reverse the action, a new button will appear: Cancel Check-in and Cancel Check-out
Read this article for more information on how to customize the colors of your reservations