The following page of the Final Fantasy 7 Remake guide has a description of Alone at Last discovery.
This event is available in Chapter 3 - Home Sweet Slum. First, you must complete all 6 side missions: Chadley's Report, Rat Problem, Nuisance in the Factory, Lost Friends, On the Prowl, Just Flew In from the Graveyard. Once you have completed all these tasks, Tifa will suggest that you go to the apartment and rest after the hard work. It is important to complete the tasks as soon as they become unlocked before Tifa temporarily leaves the team.
Outside the apartment, you meet Marle - talk to her. Now go to your room and change the filter in your sink. Then go to the next room and talk to Tifa.
Together you will decide to take a break from what is going on and go out on the town. You can choose which outfit you would like to see her in. Go back to your room.
Talk to Marle downstairs, and will you end this short event.
Reward: Crescent Moon Charm