This is a story related achievement which will unlock with natural story progression.
When you are in the sealed room on the first floor, you will open a box to gain the Dexterous achievement. Take the objects inside. When you have finished searching the room, in your inventory you will have a square metal object, a triangular jewel and a red ribbon. Combine the triangle and square, then combine this with the red ribbon. The game will not let you climb out of the window without completing this. Once you have climber out of the window, walk forward towards the front of the house again.