How do you write an ekphrastic poem? Do you just create a story for the work of art?
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.
Ekphrasis: the recreation in words of a work of art.
Ekphrastic poetry, which is poetry that verbally describes a visual work of art.