To check in passengers at the airport using the DCS program, you must first log in with your ID and password. You then enter the departure airport code and flight details like number, date, and boarding point to assign the flight. The system displays a passenger list that you can select from by number to view their data like name, nationality, bags, and more. Enter the check-in command according to their information, which completes the check-in process. To finish, enter the sign out command "BSO".