Most trick photography starts with a series of images that are layered in the same digital file. Take more photos than you think you'll need, so you can get just the right effect in your finished picture.
1. Photograph the empty backdrop.
2. Take a few dozen pictures of the ghost -- a model dressed in a white cape and a gauzy dress -- walking in front of a plain wall. Next, cut out the figures that evoke the most realistic motion and layer the images against the background photo, making each one more transparent than the last. Also make the bottom of the ghost's dress transparent so that she appears to float across the frame.