If you follow this guide exactly, it will also net you the Happy Ending? achievement.
- ALWAYS check in with Buck before beginning your shift. When you wake up, head straight to the Facility and into the Monitoring room where he sits on the Ground Floor.
- Befriend Eve by speaking with her on your Training Day (when she is on the Ground Floor Lobby, don't leave before the dialogue is over). Also, meet her in the Relaxation Room on Floor Two, Left Door of the Facility on your First Day after she leaves the note on the inside of your door.
- Fix the Generator on Day Two.
- When you arrive at Day Four, go up to Buck's Apartment (Floor Two, Room Six) and grab his Car Keys. Then, go check in with Buck at the Facility as you normally would do.
- After checking in with Buck, immediately leave and take his car. You will get into a car crash with the gangsters.
- You need to escape from Eve and the Lower Levels. As soon as you escape, head outside of the Facility and an invisible Buck will speak to you. Keep heading left, all the way to the Train Station.
- When you arrive at the Train Station, Eve will show up. Ignore her completely and just go into the Train Station and catch the train.
Do not fear if the achievement does not immediately pop, it may pop during the credits.