Instructions
Go to the dining room with the four keys in the "Mind Your Manors" chapter, and you'll watch a cutscene of Shaggy being chased around upstairs by a ghost. Defeat the Creeper in this room by hitting him with your helmet, and collect the keys. Follow the passage through the door you just unlocked with the four keys, jumping on tables along the way to keep moving along the passage.
Make a left at the broken railing and through this doorway to cross through a bedroom. In the next room, there is a huge hole in the floor, and a chandelier swinging back and forth. Catch the chandelier as it swings toward you to swing to the other side. In the next room, jump on the white button in the center of the room to unlock the final passageway.
Follow the hallway to the single door at the end. Opening this door reveals the portal that takes you to the "All Scares Upstairs" level.