Noticing Newbies

So here's the thing guys (And I don't even know if this is the right venue):

I'm an amateur writer +++ ; I have no delusions about becoming pro, I have no intentions of making money, I don't even particularly have the desire to be published.

What I do have is a story inside me. I have had it since I was a teenager. I've imagined it, I've fantasized about it, I've lived it in my dreams. I've taken online course ( through Coursera, amongst others) in an attempt to learn how to get it out of my brain.

Recently, I've started to put pen to paper. I have character profiles; I have acts and scenes labeled and chapters numbered. I have locations moved from imagination to physical paper.

How do you start writing? How do you take something you know so intimately in your imagination and put it on paper? How do you quantify this fantasy alternate reality into 90000 words?.

I am intimidated by this, but I have to get it out. I don't particularly care what other people think of it, but it needs to be up to the caliber that I have been living it for the last twenty years.

Any advice?

---> J.