Explanation: This is a central part of music arranging, so give it close attention and high creativity. Here, you become the songwriters partner.
Counterpoint is a secondary melody that complements the main melody. It can be harmony parts parallel to the main theme, in a varied rhythm or act as fills in the pauses between melodic phrases. In some instances, the counterpoint melody may be the introduction to the song, and continue to be its underpinning all the way through.
Using your knowledge of the song, identify a short melodic and rhythmic phrase that can unify the song and complement the melody. From that building block you can construct an entire counterpoint part. I suggest using a multitrack sequencer to record your counterpoint together with the main melody. Add a third track playing a soft pad with the chords of the song. Then, listen to all three tracks together to make sure there are no clashes. If there are, consider changing the counterpoint or the chords to make it work. The more interesting melodically your counterpoint is, the more engaging your musical arrangement will become.
Next: 6. Create a roadmap for the song.