Sometimes the writing comes like nothing. Sometimes I forget how I got to the ending of some stories. It brings to mind when writers say they feel like imposters. I can see where they might get this feeling. You finish something and there is time in between that and the next story and bam. There's no definite set of rules to get from A to B, but there are some tactics that work for me.
Step one: You have the story in your head? Just dump it out onto paper. Don't worry about how bad it sounds--or if things aren't in order. In fact, the worse the better. Later you will go and fix everything. Read it as though it isn't yours. Sometimes this may involve putting it aside for a bit, but in that time you can start another story. It's said that most writing gets done in the edit.
Step two: Go over the story and edit.
Now, this is what works for me. Sometimes it changes. Most of the time it's the easiest method I know. There are writers who go one page at a time (Dean Koontz). And there are writers who can just put words on paper and, for the most part, things are done. But that comes with time and experience.
Ok. Got to run. The day job is calling.