There is really no way to unlock this achievement using only information provided in game, so if you're worried about spoilers, there really aren't any. You can come back and do this after you've completed the game also, so don't worry about missing it.
The first thing you need to do is destroy the box of infinite complexity:
Once that is done, go back to the box and "call for help": 0118 999 881 999 119 7253
If successful you'll see lights start blinking on the box, This means that the box is now in "easter egg mode".
From here there are a number of codes you can enter, entering these codes will change the slide in the projector at Farley's house back on Hunrath. There are 62 different codes for 62 different slides! see this list here:
https://obduction.gamepedia.com/Easter_eggsthe easiest one to enter is: 0000 0000 0000 0000 1017
If entered successfully the lights will turn green.
Finally, go back to Farley's House on Hunrath to view the slide. The achievement should pop before you even enter the house.