Explanation: (If you're not using a piano, skip this step).
The piano is an instrument that can bridge the gap between rhythm, counterpoint, and melody. That's why it's such a great accompanying instrument. It is basically the main player in the pack, so it requires special attention.
Following your roadmap, it will either be playing a straight rhythmic vamp, underpinning the melody, or playing the counterpoint. Or, it may be some mix of the above functions. Spend time doing the piano parts, as they gives shape to the rest of the arrangement. When done, you'll want to have a written piano part so you can be sure of what to play. If you can't produce a full score, at least a lead sheet with appropriate comments should be your goal.
Next: 9. Arrange the rhythm players, guitars.