The second part of the setup for the full assassination of Sivert.
Head to the Southern side Use the large crowd to keep out of sight of the guards. More importantly, until you find and mark your target. You will find Duchesneau on this side. You should take some time to stalk him just to hear what he is up to.
He will head along the Southern side of the church and between a pair of guards into the graveyard there. Avoid the guards and head for the wall to the left of them. Avoid climbing up the wall for the moment and just stay on the outside of the graveyard. Go past the first three gates. Slip inside in the fourth one. If you hear Duchesneau talking, this is also an opportunity to get in and find a location to ambush him.
If you see Duchesneau leave the graveyard, he will walk a long patrol around the church and then repeat the loop through the graveyard.