Tips for writing atmospheric or background music:
1. The music may or may not have a melody.
2. The music will be based off on an ostinato. You can change an ostinato by adding or removing instruments playing the same part (possibly in a different range).
3. PADS, STRINGS, and SYNTHS are the instruments to use!
4. Unless the music is supposed to be in a specific style (ex. the scene takes place in a Jazz Club), you will probably not use percussion.
5. Consider using automation for volume (especially at the beginning and end of the cue).