After Bernard can access all of the Mansion in the present, go upstairs, and into the second door (the one with the R on it, on the right next to the vending machine). You can get the disappearing ink even if the door is still chained.
Run around and use the ink on five different people to get the achievement.
Make sure you squirt Weird Ed Edison before you
solve a puzzle.
I squirted Weird Ed, Edna in the room across Ed, George Washington in the past, John Hancock and Jed or Ned Edison (the sculptor and the model).