Hint 1 - hermit's house is in swamp south of Lumbridge Castle
-talk to the hermit. He'll give you the Amulet of Ghostspeak.
Hint 2 -Return to the church and go into the graveyard while wearing the amulet.
- Now you can talk with a ghost
Hint 3 - Skull can be found in catacombs under Wizard's Tower.
- You can take a skull from a cell guarded by skeleton.
Hint 4 - Return to the ghost with the skull and place the skull in the coffin.