This one confused me at first as it seems like one you would get with natural progression (and that's what other guides I found said as well) but it's not actually the case. As more people have finished the game than got this.
At the end of the game you will access a house in the gardens and meet up with Eleanor, the Marquis' wife. You'll get her mask and the ability to mirror walk, but you aren't given the chance to use it right away if you choose to continue with the story. Once you've pretty much completed the game you are given a choice to rewind the day or not, choose either as you can watch the ending and just continue your game from the menu to find anything you missed.
Once back in the main mansion you need to find and interact with one of the mirrors scattered around the house (each main area generally has one): there is one at the end of the rooms just past the CCTV camera control room in the casino area, one up near the library, one in the basement (exit the elevator room, go south and then into the left hand door in the next room), one by the theatre and one in the eastern most room on the first floor.
Interact with any mirror once you have that last mask to enter a new area (it's the walkways you can see from the basement) and get this achievement. There are two playing cards in this area for you to find as well.
TL:DR once you get Eleanor's mask use it on one of the mirrors the servants use to get around in the mansion. Sorry for the longer description - wanted people to know where to go.