When you awaken on the next day, Loudred will call you over after the morning meeting. He says that since Diglett can't do the Sentry Duty due to his father not being able to change the Job Bulletin Board and the Outlaw Notice Board, you have to take over. After this, Diglett apologizes and says how important changing the boards are. He leaves, and Loudred gives a detailed explanation of what Sentry Duty is. He will ask whether you understood or not, and if you picked "UNDERSTOOD!" then you may proceed on to the Sentry Duty.
As a footprint comes up, the touch screen will show four Pokémon, only their names. If it takes a bit longer, they will give a hint and a picture. As you can only get three wrong, don't answer too quickly and look at the hints and the pictures if you're still unsure. However, there is a time limit and if it takes too long Loudred will mark it as wrong. After getting all six done, the day's work will be over and you head over to Chatot to see your results. He will reward you depending on your score.