Question

How do you write an ekphrastic poem? Do you just create a story for the work of art?

Answer 1

Not quite. A story would be a new piece of art inspired by the original, but not necessarily describing it. To be "ekphrasis" it must be a word description of the original art.

Explanation:

Ekphrasis: the recreation in words of a work of art.
Ekphrastic poetry, which is poetry that verbally describes a visual work of art.
https://study.com/academy/lesson/ekphrastic-poetry-definition-examples.html

Best examples:
https://www.theguardian.com/books/2009/nov/14/ten-best-ekphrasis-john-mullan