Different ways you could go about doing it depending on what you want the ghosts to be like. For instance if you wanted the ghost to pop out of a grave stone then vanish I would make a ghost model, tie it to a prop_dynamic that is nonsolid and parented to a func_tracktrain that follows a path_track through the headstone at a fast speed. Then on the last of the path_track have an output fire that kills the prop_dynamic and func_tracktrain so that it just quickly pops out the disapears.

And how do i "tie" stuff together? Im new so please bare with me.

Set the prop_dynamics model variable to the model.

Here's a tutorial video for something similar, just make the prop_dynamic and have the path go through a grave stone.