On this page of the Assassin's Creed Valhalla Siege of Paris game guide, you will find a detailed walkthrough of The Ghost of Saint Germain side quest.
Starting the mission
You will come across this mission while traveling through the open space, in the western part of Paris.
On your way, you will come across several residents of the area. They will direct you to the church, which is the source of their worry.
The church is within walking distance. All you have to do is run to the building, go to the very top and read the note that lies in front of the dead man. This will complete the mission.