Time to get inside Notre Dame. This is very easily done using the Underground entrance at the Southeastern corner of Notre Dame. You need get inside the tunnels.
Drop down the ladder and head around the corner. Head past the first divide in the tunnel. Hug the left-hand side of the tunnel. You will find a door just past the split. Head inside it.
Go up the stairs inside the doorway. Go to the first landing and hit the stone button to the left of the next staircase. You will open the way into the church above. Head into cover over to the right and start moving carefully. This will also enable the 2 optional objectives.
Slip up the stairs after the nearby guard has moved away or is facing away from the top. Go over to the right and take cover behind the pillar to the immediate right of the stairs you came up. Watch the nearby guard. Wait him to come over to you. You can get an easy cover assassination. He can call in another nearby guard if he notices you. Continue over to the East. either slip by these guards or take them out using cover assassinations.
Once on the Northern side of the Church, you will see Sivert enter a confessional. Use the nearby hay cart for cover and the Last Known Position to lure away at least one of the guards. This will clear the way for a simple way into the confessional. Just be sure the other guard doesn't notice you (which is unlikely). If he does, use the last known position feature to lure him over to the nearby haycart for a quick kill. Just head into the confessional after that.