I don't have any step-by-step photos, but it was really easy.
Here is the shirt I started with:
After cutting the shirt along the crochet piece to make arm holes for the vest, I sliced open the back of the shirt (which is now the front of the vest) and attached the fabric from the sleeves, just to make the vest a little more drapey. Does that make sense? Well, anyway, here's the final product:
Sorry, no photo of the front of the vest. Maybe one day I'll add that.