Received from: automatically during Where is Triss Merigold? In case of helping Roche in The Assassins of Kings.
Step 1: Follow Dandelion to the lower floor
Run out of the room above the tavern and head into the tavern itself. Go to the lower level (the stairs are right beside the entrance, on the right) and go to the next room. In one of the rooms you will find Margot hanged. The mission will end. If you search the room, you will find Margot's notes.