The achievement requires you to find three easter eggs: The song "Burning in the Third Degree," the severed hand of a T-800 and the blueprints for the HK Centurion. You start off in the south-westernmost corner of the map. The guide at the end, written by Steam user Hanni, has a map to each of their locations, assuming my directions aren't clear, as well as pictures of what to look for.
1) Burning in the Third Degree - From spawn, head due north, towards the tunnel linking the northern map area with the south. Follow the tunnel north, hugging the western (left) wall after the first turn. Following the wall, you should find yourself in a small restaurant. Behind the counter is an interactable radio. Activate it to play the song.
2) T-800 hand - Again, head north from spawn, towards the tunnel. However, this time head east instead of entering the tunnel. Eventually, you should reach a hospital (if you've completed Chapter 11 of the main campaign, the same hospital you found yourself in near where the T-47 was patrolling). Head to the second floor and jump out the window of the third room on your left. Following the wall to the right, there will be another room, with the hand standing on an altar. Interact with the hand for the next secret towards the achievement.
3) Centurion plans - Head north from spawn a final time, but this time, at the crossroads in the tunnel, turn right. Follow the path into another tunnel, which will head north towards an abandoned factory. In the easternmost part of this factory, just behind some stairs, is a destructible wall. Break it down to find two boards - one marking the location of the T-47's weak spot and the other covered up. Interact with the latter to pull the cover off and secure the third secret.
Find all three during a single run and the achievement is yours.
https://steamcommunity.com/sharedfiles/filedetails/?id=19135...