For a real description, focus on this in Sequence 5, Mission 1 called "The Silversmith". Your mission is to find Germain, but in that process you have a secondary mission of lock-picking four doors. They are all only one lock, which is as easy as they get. I'm also fairly positive there is a fifth door in the building so you can do the whole achievement here. If you're missing one, you're bound to stumble across at least one in the main missions before this point.