After the conversation in the inn, head to the man called Pitcairn together with Charles.
There you will witness a quarrel which will end with the soldier you're looking for getting imprisoned.
Optional objective: Limit detection time to 15 seconds or less
You will now have to follow his escort while making sure not to be detected and not losing the sight of him for more than 15 seconds.
If one of the soldiers notices you, a yellow triangle will appear above his head, eventually changing to red. If you want to complete the additional objective, you cannot let the guards look at you for more than 15 seconds. The total time of all the enemies stacks up. If the red triangle fills up completely, you will fail the mission.
Follow the group while trying to keep distance and hide behind walls and between groups of people.
Eventually you will reach a place where Lee will draw the enemies' attention and start running away. Chase him.
Optional objective: Perform sequential combo kills - 3.
Optional objective: Prevent snitches from calling for reinforcements.
Optional objective: Completed all the constraints in one playthrough.
After reaching the destination, you will have a short conversation and a fight with British soldiers. The fight itself isn't hard, but you will have to focus if you want to complete the optional objectives.
The first you have to do is kill the enemy marked with a red dot with a white border. That way you will prevent him from calling in reinforcements.
The second objective is performing a combo of three kills. In order to do that, you need to lead out a counterattack which will kill the enemy and afterwards quickly press the attack button while leaning the analog stick towards another soldier. If you see that someone's trying to attack you, parry him to continue the combo.
After the last soldiers is killed, approach Braddock and end the mission.