Pick up the Marty Bullfinch holotape beside the filing cabinets inside Nick Valentine's office in Diamond City and play it to start this quest.
Head to the Faneuil Hall and find your way to the rooftop. The grasshopper should be on the tip of the yellow dome. Collect it and read the note “Food for the Grasshopper” in your Inventory > Misc option. Next, you have to find Shem Drowne's grave.
Fast-travel to Pickman's Gallery then head west while following the quest marker. When you reach the small cemetery, dig up the grave to complete the quest. Don't forget to loot his remains, including a unique revolutionary sword.