The assignment you are being asked to complete is an exercise called "ekphrasis." It has been a standard part of the writing curriculum since antiquity, when it was part of the progymnasmata, or "preliminary exercises", students preformed as propaedeutic to their rhetorical studies. Perhaps the most famous example of ekphrasis in literature is the description of the shield of Achilles in Homer's Iliad (18.468-607 ) which begins:
First of all he forged a shield that was huge and heavy,
elaborating it about, and threw around it a shining
triple rim that glittered, and the shield strap was cast of silver.
There are many other examples of this sort of description in literature. An essay frequently used as a model for students to imitate in learning how to use description is E. B. White's "Once More to the Lake."