Supervising someone at work

I’ve never had to supervise someone at work before but in the last few years we created a position that I am in charge of. The person we had previously in the job left, and now we have to hire someone new. Part of why they left was for personal reasons, but probably some part was the way the position was structured without good accountability or support. I feel very overwhelmed with the prospect of adjusting the job description/responsibilities, hiring someone new, training them, managing them, etc. It is a position that straddles several departments so it is not clear or intuitive the best way to set things up.  Here is my model:

C: New position is created
T: I don’t know how to figure out this new position and manage them
F: Overwhelmed
A: Procrastinate, avoid working on it, do half-hearted attempts, just assume “it will all work out” but don’t really put in the time for planning/troubleshooting
R: The position fails

I see that this way of thinking is setting me up for failure. Can you help me come up with a model that will set me up for success?