Video Description:

Thief - Quickly Pick a Lucky Lock - Pick a lock with masterful speed [Bronze / 10G]

You can do this at any door in the game that has 3 pins. Save your game before interacting with the door. Now try to pick the lock and remember the correct combination. Before unlocking the third pin, reload your save game. You'll be back in front of the door and just need to enter the same combination as before. Do this in under 3 seconds and the trophy / achievement should unlock.
If you fail, you can reload your save game every time.

Note: If you've already opened the door shown in the video, you can return to the clock tower (your hideout) and it will get locked again.