I wrote a design document, including PlantUML diagrams, and mockups, and hyperlinks to designs I liked and specs I wanted to cover and architectural/quality concepts I wanted to include, and coding guideline stuff from our PO. It was like 5 pages long.

And then I had it generate the .feature files, and went through multiple iterations on each file, editing and then having it review-and-suggest-more, and basta. The team has been feeding me their wishes, on the side, in Gherkin, and I just say "Please incorporate this into the .feature files and then implement." and ta-da!