At the start of the mission you must go up an elevator to reach a checkpoint. Kill the guards in this room. Take off your sunglasses if you wearing them. When you walk in the next room, the sun will be shining in your eyes. Flip your glasses down and you can see guards shooting out the windows you were looking at. Go to the right as soon as you walk in. Try not to draw too much attention. If you keep going to the right you will hit a window. An icon should appear at the bottom of the screen, indicating that you can use something. Press X and Sam will get out a small black tool, and use it to cut a circle in the window. If you push forward on the window, Sam will put his legs through and squeeze through the hole so that he is now outside.