Decide what theme will inspire your scary music. Some favorites are stormy weather, insects, graveyards, blood and supernatural beings.
Write music in C minor or F minor keys as they are crucial in creating a scary atmosphere. This is particularly so when playing those chords lower on stringed instruments and the piano. Minor chords suggests sad or scary feelings, while major chords evoke a happier and brighter mood.
Play some minor keys on a guitar or piano. If you use a guitar, make sure to let the string slide. Keep the sustain pedal depressed all the time if you play the piano. Doing so creates haunting feelings in the audience.
Add a high-pitched solo female vocal to impart an eerie feeling to the listeners. You can also add barely audible footsteps or record panting sounds.
Create unusual sounds using everyday objects. For example, use clattering wooden chopsticks to simulate bones clacking. Incorporate loud percussion noises to surprise your listeners.
Add dissonance to create tension in the music. You can achieve this with cluster chords, groups of minor notes that scatter in your music. An example of a minor notes group would be F-Eb-D-Db-Fm.
Record your finished scary music and load it into a computer music making program. Use the software to add special spooky sound effects or remix your composition. You can fine-tune your music with the software's MIDI playback and mixing functions.