Whitch version do you have? And you mean how do you catch them? You know that ghosts appear depending on chance and you have opened certain contracts (you must have read the guide). Unless you have a ghost lamp obtained from the Thundergirl quest. Make sure you can use cheats (the cheat list is available in the walkthrough). The fastest way to catch the ghost is to complete the Thundergirl quest / Rosuthundra quest first. You can force the ghost to appear in the contract you choose in the contract options, then start searching for the ghost's whereabouts, then bring the ghost weakness tool and catch the ghost when the ghost is not hunting.