Watch the movie. If you have the chance to watch it without any music or temp score, do so. This will allow you to interpret the action on the screen as you experience it. Consider the genre of the movie as you watch it. Take notes of key moments and emotional scenes.
Develop your ideas. Just as music is an emotional experience, movies operate the same way. Be aware of the tone and emotional mood of the movie. Consider movies of a similar genre and what the score to those films sound like.
Write the music as you watch the movie again. This is where knowledge of music theory can be helpful. When composing film music keep the key, chords and mode in mind. Each genre of movie has a general sound and mood associated with it. Create your own score but stay in the realm of what is accepted in the movie's genre.