Make your way out to the Frontier and you'll find George Washington stationed in a fort out to the west. Talk to him and he'll tell you about some supplies that have been stolen. He'll tell you to seek out a Mr. Church. Head to the nearby chapel and go inside.
After the cutscene, walk out the front door and use Eagle Vision to track Mr. Church's men. Inspect the boxes and then make for the green circle to your left. When you reach the road, search again for wheel tracks in the ground. Follow this road and it will lead you to the man you seek. Keep in mind that your companion can't run through the trees like you can so you'll have to keep to the ground when traveling.
You'll find the cart driver Mr. Church and when you do, he'll take off running. Chase after him like you did Thomas Hickey. He will be much easier to catch so you should have no problem doing it within the time limit. After catching him, go to the nearby green marker to find a cart being drawn down the road. Wait for it to start moving and then cross the road. Climb up the rocks and make your way into the trees. Use the trees to follow the cart without being seen. Remember that you can't jump into the cart because of a constraint. You'll eventually end up back on the left side and see a guard on the ground near a pile of hay. Do a leap of faith in the hay, kill him, and then wait for the cutscene.
When you regain control, you'll be in the hay cart regardless of where you left off. A guard will walk over to investigate the hay in the cart. Kill him and then hop out of the cart to the right. Walk along the right side of the road and then when watch for a patch of bushes on the left side of the road that you can sneak into. Just past those will be a stack of boxes.
Stand behind the boxes and wait for the men to approach a building. One of them will go in but another in front of the house will continue walking along the road. They will continue to follow the road to the left. Wait for them to get a little lead and then start following them just off the left side of the road. You'll find another set of tall boxes that form a corner and a guard will be crouching over working on something. Kill him and then take up corner cover on the left side of these boxes.
Wait here for moment and find the man just ahead by the next boxes. The two men talking will stop for a moment next to this man. Wait for them to resume their walk and kill this man as well. Continue to tail the two men. They will come to a slightly broken down building ahead.
Haytham will get caught by some guards and you'll have to save him. Attack the mercenaries that are holding him to start a fight and free him. He will fight with you temporarily but then he will leave you to fight the rest on your own while he heads for New York. There are a lot of tougher enemies in this group so you will have to use special counters like disarming and throwing to kill these enemies. Alternatively, you could always shoot them to get rid of them quickly.
