Talk to Dr Fenkenstrain at his castle to the northeast of Canifis. (1-4-4)
Equip your Ghostspeak amulet or Cramulet
Talk to Gardener Ghost who is in the Northern part of the castle
Ask him what happened to his head.
Lead him south to the Haunted Woods. (He will teleport to you when he becomes out of range)
Dig at graves until Decapitated Head is found. You can talk to him to get directions.
Head back to Canifis pub and purchase the Pickled Brain by trying to take it or use Telekinetic Grab to grab it from the table. It's much cheaper to just buy it.However, telekinetic grabbing the brain completes a medium level task for Morytania.
Use the Brain on the Head.
Arms, legs and torso
Go back to the castle and go up stairs.
Search the west bookcase for The Joy of Gravedigging and you will find a marble amulet.
Search the east bookcase for the Handy Maggot Avoidance Techniques and you will find the obsidian amulet.
Use the marble amulet on the obsidian amulet to get the star amulet.
Head east of the castle to the tombstones.
Use the star amulet on the far east tombstone, push open and climb down.