This achievement is unlocked during case 2 A Study in Green - Midway into the case you will visit the Bernard Marleys Foundry at night.
You goal is to use a variety of mechanisms in order to access Marleys office on the upper floor. Use RB to highlight objects you are able to interact with:
The following is a guide I have wrote. I tried to be as concise as possible in as few words. I hope it helps.
1. On wall with 3 levers, Go to middle lever and drop down the metal cage onto the cart.
2. Go right across to the far side of the room where you will see a single lever. Interact and move the gate Right
3. Go to Lever One (nearest to office), move as far right as possible.
4. As Holmes head to the corner of the room (Right corner from wooden entrance doors) Climb up a large metal fence which acts as ladder, make your way across a beam to a ladder (taking you to the rafters) ascend the ladder. walk across another beam onto a small metal platform. There will be a ladder here (hard to see) that you need to descend. Now you can walk across the metal cage to the otherside.
5. Press LT to switch to Watson - Move the cart onto the opposite tracks so that Holmes can walk across the gap stopping him. You will need to do a U turn. In the middle of the room there is a lever which turns parts of the tracks. When the cart is in position.
6 As Watson - Go to the 3rd lever (nearest the Horse Statue). Interact with it and move it as far left as possible
7. Switch to Holmes - Walk over the cage and make your way to a long beam. You will need to balance. I found it easier to wobble the thumb sticks gently once they were in place, rather than trying to keep them perfectly still. Trust me it works. Once you get across the beam, interact with the chain. 3 swings back & forth then press X to jump
The achievement unlocks as soon as you enter the office