You do not feel comfortable to write specifications from ground up, I understand that.
In the near futures, you will receive only slightly detailed specs and you will be asked to give feedback and advices to story owners to help them improve theirs documents effectiveness instead. Please, use the skills of yours as a technical expert to tell the story owner (once you have a clear view of the intentions) if you have a better idea: do not hesitate to disagree and say why.
You are engineers, you should give your expert point of view when interacting with someone!