A Clean Plate: Film your footage on a tripod without your ghost.
Film your ghost on a green screen or a black screen(removing the black makes it transparent).
Key your footage, and place your ghost actor in your scene.
Now reduce the transparency/Opacity (Same Thing) on your keyed ghost actor using After Effects/NLE.
Add an external glow (Pick a color, for the light side you pick BLUE, dark side pick RED.)
You now have created your the most classic ghost effect!