You begin the mission on the rooftop of the East Wing. Walk forward and look find the chimney cowl (the iron/metal object sticking up) and you should see a 'rappel' option appear in the interaction menu. Activate it, then rappel down to the open window below. Head inside and stick to the shadows until a guard enters the room. Once he heads to the bookshelf, knock him out and head towards the door he came in through, avoiding the surveillance camera. Open the door, but avoid exposing yourself inside of the hallway. Peek the camera around the door and you should see two guards: one patrolling and one standing, facing away from you. Wait for the patrolling guard to turn around and move towards the stairwell door to the left. Open the door and immediately cut left, heading down the stairs. (Yo