When I saw the name of this Google Maps application I hoped that the creators had managed to attach GPS devices to a number of ghosts and were tracking their movements in real-time. No such luck. Ghostag is instead a crowd-sourced map of ghost sightings.
To tag a ghost on the map you simply need to click on a location and add the details of your sighting. If you manage to get a picture of the ghost (or failing that - the location of the sighting) you can also add that to the map.
The map uses the Google Maps API v3 Styled Maps function to create a suitably creepy look. When viewing the map turn on your speakers as it also contains some ghostly sounds and the occasional ghost like apparition appearing on top of the map.