You need talk to Nutshade, he's in the BrambleClan borders and first, you need to talk Breezestar, she's in the FireClan leader den. Then talk to Nutshade again. As you can Finished the dialogue. Go talk to Juniperpaw, nutshade was talking about. Go talk to Nutshade again. Type any custom sentence with "ghosts" word in there. Then. He says about New Moon. The weather needs to be cloudy and afternoon. Then go to the dead land in FireClan medicine cat den. As you can see Ghost Lucy in that land. You made it!
I hope, this helps!