It's very simple.
And keep writing until it's done.
No more, no less.
With all the writing advice out there, this one goes to the heart of much of most writing. Some writers try to push a short story idea to something longer just for the sake of making it longer. Some writers even try to squeeze a longer piece into something shorter. Again, just for the sake of making it shorter.
One of my recent publications titled Bruce is an example of this. When the story was originally conceived, it was a short story. A few thousand words at best. By the time it was complete, it was just shy of 11,000 words. Not a short story at all. A novelette. But I wrote the story that needed to be told. I didn't cram anything in that didn't pertain to the story and I didn't leave anything out for the sake of keeping it short.
I wrote until it was done.
Try it. Write a story without any notion of length. There are a number of epic fantasy novels, many published a few decades ago, that were bloated just to fit the genre. Stephen King has even written a few door stops--although we can argue if the door stops worked or not. People have argued that the last 2-3 Harry Potter books could've been cut down too.