For this achievement you have to join the Railroad faction. Here are all steps:
1 - listening in Diamond City random people which talk about the railroad rumors to start the quest
2 - go to the quest marker from the quest (Park Street Station)
3 - follow the lanterns or the red line to solve the puzzle but both is not necessary, you can directly go to the Old North Church
4 - go inside the Old North Church but watch out some enemies are here
5 - go to the basement and enter the password "railroad" to open the hidden door
6 - talk with the woman and with Deacon, start the next quest called "Tradecraft"
7 - complete the quest (ca 30 minutes) and at the end you will come back to the room
8 - talk with Deacon and the woman again and use the "Join the Railroad" option
PS: During the mainstory quest "The Molecular Level" you can also solve the "Road to Freedom" quest. The main quest have the same steps (2-5)