Think of or find a picture of a dog. The young man is in a garret (a room in the attic of a house), so write where the dog is (park, sidewalk, alley, etc.). The man is leaning over a desk. What is the dog doing? The desk is covered with papers, and holds a bunch of withered violets and a glass. What objects are near the dog? A water bowl and a food bowl? A chew toy? Garbage? Maybe the dog wears a collar and tags. Does he or she wear a harness? Wilde described the man's hair, lips and eyes. What is the expression on the dog's face? The body language? The length and colour of coat. Is the coat dirty or clean?
I can't remember "The Happy Prince", so I don't know anything about the young man. I assume from the description that he is either a romanric poet or he is trying to compose a persuasive love letter. His eyes are dreamy and he has kept withered violets on his desk. I dismissed the idea that he was depressed and drunk (the tumbler) because his eyes are dreamy.