You'll have split up with the mysterious woman after the last mission. Head towards Braddock's camp to meet up with her again. When you find her, she'll recommend that you use the snow storm to your advantage when you sneak into the camp. Make your way to the east and you'll find a wagon headed for the base. Slip into the back of the wagon and it will take you right into the camp.
Inside the camp, the wagon will stop and you'll be able to get out undetected. You'll find yourself in some bushes and there will be a couple of red coats nearby talking. From these shrubs, look south and find the next set of shrubs. To your left will be a patrol walking up and down the road. Wait for him to walk past your location and run to the next set of shrubs to listen to the soldiers talk.
They will walk back along the road that the patrol is on. They will walk past you and then the patrol will return going in the opposite direction. When he passes you again, run back to the previous set of shrubbery. The talking soldiers will turn the corner and you can follow them around the tree to your right. As they approach the wall, cross the road to the haystack. You want to do this as soon as they pass the hay because the patrol is going to come back up the road towards you. Get into the haystack and wait for them to start climbing the stairs.
When they are about halfway up the stairs along the wall, hop out of the hay and into the shrubs at the base of the ladder. Once they pass the ladder, start climbing to the top. At the top, look left and you'll see more shrubs to the left of the road that run all the way along the wall and up to a hay cart and tent. Jump into these shrubs and make your way along them to stay in range of the soldiers. Soon they will stop talking and you'll be done with them.
The next bit gets a little tough. One of the constraints for this mission is to not kill any of the guards but you can also not be detected or the you are automatically desynchronized. Try to get back to the shrubs you landed in when you exited the wagon. From here, you want to make your way across to the next shrugs. Just beyond these shrubs is a path that leads down to the first canon. At the base of the stairs on the left will be more shrubs. Get into those shrubs and watch the man who guards the canon. He will approach the canon, and the he will walk back towards the shrubs on your right. When he walks far enough to get back to those shrubs, make for the canon and disarm it before returning back to these shrubs.
From here, head back up these stairs and then up the next set of stairs in front of you. At the top of the stairs will be more shrubs just to your left. Ahead and to your left will be the tent with the map in it but the guard in front is pretty vigilant. There is also the canon to the right and a guard on the platform above it. To get the map, you need to get behind the tent here and whistle to the guard watching the map. As he comes towards you, slink back into the shrubs so he can't see you and when you have a chance, dart for the map and then head for the shrubs to the left of the tent (or the shrubs to the right if you think you were taking too long). If you don't see a good opening, wait for him to leave and try again.
You should have one canon down, the map, and be next to the tent that held it. You want to get back to the shrubs at the top of the stairs. Try to get there unnoticed by watching the guard on the platform above and timing him against the guard who should have returned to in front of the tent. When you have the chance, get back to those shrubs. From here, you only have to watch the guard that is watching the canon. When he turns to walk away from the canon, go up to it and disarm it. As soon as you do, jump over the wall that the canon rests on and make your way along the cliff face as best you can. When you can't go any further, jump into the water and swim away. It will take you a little longer to get where you are going than going through the front gate but it is much safer than trying to walk past the three guards in front of the gate.
When you return to your mystery woman, you'll have finished the mission.
