The job description is the last thing you should write, not the first. A JD assembled from a wishlist of skills attracts generalists and repels the specialists you actually need.
Start with the work, not the title. What are the three most important things this person will ship in their first six months? If you can't name them concretely, the role isn't ready to hire for yet.
Then pressure-test the level. Founders routinely over-spec ('we need a staff engineer') when they actually need someone hungry and high-ownership a level down — and can't afford the staff comp anyway. Be honest about the trade between seniority and budget.
Next, decide what this hire is allowed to own versus what stays with the founders. Ambiguity here is the number-one cause of a strong hire leaving within a year.
Only once those are clear should you write the JD — and at that point it nearly writes itself.