Take notes while watching. Note all of the major characters and any minor ones who make an impression on you. Also note plot twists and other events, along with your initial impressions of them. You may need to watch the movie twice or even more to get all the relevant information you need.
Review your notes and dig into the plot. Look for any life lessons, insider comments or jokes that the reader would find interesting. You should also consider the quality of the acting and any directorial decisions (this could be anything from the color of an outfit to the camera angle of a particular scene.)
Write an outline that takes all of the major points from your research and then compose a rough draft that hits all of those points, along with any interesting details you found.
Review and edit your rough draft as often as necessary to ensure it is of high quality before submitting it.