Delivering a taste of his own medicine.
At the start of the mission, head towards the waypoint on your map and speak to the marked man to begin your task. After the conversation, Jacob will have to chase after a merchant. Don’t worry too much about catching him early on, as he’ll lock gates behind him as he runs along. Once you reach the sewer area, just navigate your way through the only path that’s available to you. You’ll only be able to tackle the merchant once you’re out of the sewer and on street level again.
Head for the next waypoint that shows up on your map, and then use Eagle Vision to look for your target. You should see your target getting into a carriage as he departs the area. Follow the carriage from a distance and wait for it to stop. Once the carriage stops, steal the plans from your target.
Finally, head to the third waypoint on your map. You’ll want to use the Rope Launcher to reach the higher ground in this area. If you want to complete the optional objective of approaching the target undetected, I suggest using throwing knives or air assassinations against any guards that might spot you. Once you’ve taken care of them, simply sneak up behind and kidnap your target. This will complete the mission.