Start a new game. Get the key from under the Christmas Duck in the cupboard on the left, and unlock the door. Go up the stairs, then into Sam's room. Unlock her locker with the following combination:
Take the basement key next to the picture on the inside of the locker door. Go down to the Servant's Quarters in the basement. Proceed through the room to the corridor beyond, where you'll find a safe sitting in the corner. Unlock it with the following combination:
And the achievement will unlock!