To write a narrative description of a smart person, you first need to understand what a narrative description is:
A narrative description is an accurate and detailed description of an individual or item. There are certain stylistic formats that narrative descriptions tend to adhere to, but the main requirement is that it is a true-to-life accurate written portrayal.
If you were to write a narrative description of a smart person, you would methodically identify and list the attributes, both physical and intangible, that are characteristic of a smart person.
While this subject matter can be fairly subjective, a narrative description attempts to take a more objective approach to the written description.