Consider what purpose the background music serves. Background music that is meant to play in a dentist's office will sound much different from background music for an action movie.
Choose a notation or audio editing program on which to write the background music. Finale and Sibelius are the two major music notation software programs. Audio programs used to make background music include Reason, ProTools, Cubase and Audacity.
Determine what sounds, if any, will be played over your background music. Sounds can include character dialogue or noise from the movie scene that will play the music. Knowing what will sound during the background music can help you choose the sounds you want in your music.
Decide which instruments your background music will use. Choices for instruments include digital instruments and analog instruments. To help choose instruments, listen carefully to a piece of music that is similar to what you want in the background music.
Choose a main theme or two. A main theme is a short melody that recurs throughout the background piece. Experiment with varying the themes in different ways until you find a pattern that has the musical feel you want.
Use a notation program to orchestrate the main theme among the instruments you chose. With live instruments, you can print the notation score to give to the conductor and musicians who will perform the piece for recording. If you are using digital instruments, you can output the notation sound to a musical instrument digital interface (MIDI) file to play back the notation with different sounds. A MIDI file controls digital instruments; you can assign different sounds to MIDI files by importing them into a music program that handles MIDI.