[ it's a pity eyerolls aren't audible, because Strange is getting an epic one for that. Getting into getting rid of ghosts is a much more interesting topic. ]

It depends, but the general principle is the same. The basic ingredient to create a ghost is death -- usually a violent one. Sometimes banishing a ghost can as simple as finding the bones and giving them a proper burial. In other cases, there was a lot of suffering, or there's some unfinished business tying them to this world. They can't tell you what it is, though. All they feel and know is pain.