Missable achievement but easy to get.
You'll get this during your second trip to the mansion, which is quite late in the game. There are a total of eight masks, all are displayed in glass cabinets and are very easy to find. They are spread between different rooms.
Two are upstairs at the top of the stairs, the other six are downstairs in adjoining rooms. At the foot of the stairs take the second door on the left, where you'll find four masks and the only other unlocked door in this room takes you to the last two.