I'm sure Chris will be along with sage advice as IIRC he's belongs to a writers guild.
I've written some stories both short and long. The long ones I tended to do it like any project: Top down design.
I wrote down the chapter names first, then roughed out what was to happen in each chapter, then in more detail and then finally the dialog and descriptive stuff
You'll need a start where the characters are introduced and also some of the relationships between them, then a middle which usually consists of them getting into some perilous situation and then the ending where they get out in some kind of climactic way. The story also needs to be carefully paced.
I do have an unpublished trilogy (Trial of faith, Consorts quest, Plane dispute) subtitled: Extracts from the chonicles of the pure city. each book runs to about 100-150K words and those that have read it think that the length is about right. Sword and sorcery stuff in a post nuclear world (about 5 thousand years after the event, the glass deserts on one of the continents are just beginning to weather) but with a special twist.
Having written it there's enough extra material for a sequel so like D'Adams it might be in four parts.
One thing, you'll need to re-read it many times and re-edit endlessly. I still go back and amend things even now. When you're happy then get someone to beta read it. They'll see things that you never even thought of.
I do also write some fan fiction for the Gundam Wing characters. This is much easier as all the fans know exactly how the characters look and act so one can concentrate on the settings and story.