We can compose the majority of our piece with three instruments only. Even if we end up with a total of 6 instruments it might be best to compose only for 3 instruments so that we can keep track of the harmonic structure.
There is a simple way to go quickly from 2 instruments to 3 instruments. Try doing this with your piece:
Great! Now what....
You now have two instruments playing the same exact background part. We say these parts are in unison. You can quickly make harmony from unison parts! Try this:
You'll likely need to double check your harmony parts with the main melody and make a few adjustments. For example, look for any open intervals or any clashes.
Visit this topic about filling out the orchestration here: Talk about Orchestration!
Share your thoughts and experiences with this technique. How have your students performed using this technique in their piece.