Watch the movie or play for which you will be creating the music. Write down your impressions and what type of music you think would be a good fit for each scene. If possible, mark places (in minutes) where the music should swell, decrease in volume or have some other type of effect.
Using the information you gathered from viewing the performance, choose a key signature. Minor keys generally sound sadder or more ominous than major keys. Harmonic minor, melodic minor and pentatonic sound exotic or foreign to most people. You can choose to use different musical modes to create an eerie or strange feel.
Choose a time signature and tempo. Both can change at any time, but the introductory passage will be representative of the entire work. It is important to choose something that fits with the theme of the performance. Time signatures such as 3/4, 4/4 and 2/4 may sound like marches or “typical” classical music. Six/eight often sounds like a dance, while odd compound signatures such as 5/8 tend to be more jazzy and complex.
Think about orchestration. Any exciting or grand parts will sound best with more instruments, especially brasses. A love scene, on the other hand, might sound best with just a string quartet. Orchestration greatly affects how your music is perceived.
Begin writing the music. Write things in standard notation, and do so neatly. Make sure that the music lasts for the length of the production. The best way to do this is to play one of the parts yourself and time it.
Record the music if you don’t have a live ensemble to perform it at the event, or if it’s for a movie. The best way to do this is to hire a professional to record it. If you cannot do that, then it is possible to record the music yourself on your computer. You need microphones that connect to your computer, in addition to a free program such as Audacity (see the Resources section), or a paid program such as Garageband or Pro Tools.