You need to be at Adventure Rank 30 to unlock this quest. To start it, talk to Aramis that's hanging around in front of the church in Mondstadt. He will ask you to find the lost posters and billboard.
3 of the locations are marked on the map shown above. The 4th location is on the roof of the crafting table.
The first one you can reach easily by gliding southeast from the teleport waypoint that's on top of the Knights of Favonius building.
Keep going east from the first location, and go to the building that's close to the blacksmith. You will see a small marker above the poster when you're close.
Third location is the billboard that's all the way north of the city, behind Angel's Share (the tavern), up on the border of the wall.
Final location is the roof of the crafting table building.
Once you're done with all 4, head back to Aramis to complete the quest. You will get 6 Hero's Wit, 5 Northern Smoked Chicken, and 20,000 Mora.
Puzzles, Secrets, and Hidden Chests in Dragonspine
ATK% VS Physical/Elemental DMG% in Genshin
Puzzles, Secrets, and Hidden Chests in Liyue