A descriptive analysis essay is different than a strict harmonic analysis essay, or even that pertaining to form. A descriptive analysis essay focuses on the whole piece and how sections, themes, rhythm, harmony, history, practice and composer's preference play on one another.
A descriptive analysis is a survey of a piece, rather than a chord by chord analysis so, in addition to a harmonic analysis, there are several other factors that should fit into a good descriptive analysis. When you are presenting a descriptive analysis, keep the listener in mind, not the theory professor. Think about the elements of a piece that the listener would find important and, especially when it comes to your harmonic analysis, what they would hear. Below is a sequence of how I perform a descriptive analysis, and how I would structure my paper:
1. Identify the historical period of the composer and it's typical characteristics, and at least 3 facts pertaining to the composer's experience with the medium of the piece. For example: if the piece is a Beethoven Piano Sonata, then I would identify Beethoven's dates, the typical characteristics of the style (Classical, Romantic, etc.), the date-range of the composition of that piece, which creative/style period of ...
A descriptive analysis is modeled.