Watch the film for which you are composing music. This will give you an idea, stylistically, of what type of music you should compose. For example, if the movie is about a spooky, supernatural event, you might want to write a haunting theme with cellos and piano. If the movie is set in Mexico, you might try adding some Spanish guitar to the mix.
Create a map of the film score that details where the film needs music and where it does not. Music in scenes with heavy dialogue might be distracting. Concentrate on scenes with lots of drama, action and intensity.
Upload the film into your computer's recording program. This will allow you to sync the film with the music. Cubase is a good recording program recommended by Music Master Studios.
Connect the live instruments to the recording equipment. You can also add MIDI sounds to the recording program if you do not have live instruments available. You can arrange these computerized sounds into a musical composition.
Play the music alongside each scene as the movie plays. The music needs to fit the mood of the scene as well as ebb and flow with the action. For example, if the scene features intense action such as a car chase, you'll need the music to move quickly, perhaps by adding drums. Sad scenes require slower music to invite the audience into that emotion.