Added to what Chris said: (example) Sailboat on top timeline….
the shot you want it in on the timeline below it.
Then use the mask tool to draw an outline around the sailboat, and
immediately you will see it composited on the bottom layer,
from there now you do some more tweaking and feathering and
then you can even keyframe the sailboat movements.
Steve Rhoden
(Cow Leader)
Creative Arts Director and Film Maker.
Portfolio at:
http://www.youtube.com/hentys