First, write a book. Then, realize there’s a perfect part of said book that could be expanded into a short story.
Sketch out a short outline. Think: this will be easy. Feel: this will be hard.
Finish revising your book. Come back to the short story idea. Realize you kinda maybe need to write it because what happens in it will largely inform your next book. Check your schedule. Find some time to commit wholeheartedly. Two weeks should be enough. It’s only a short story, right?
After one week, google: how long is a short story supposed to be anyway?
After two weeks, google: how long is too long for a short story?
After three weeks, google: how long is a novella?
After four weeks, google: why is everything I touch a novel?
Set it aside unfinished because you have other projects with Actual Deadlines. Hit those deadlines. Celebrate.
Gradually become aware of a general sense of unease. Realize the short story is watching. Waiting.
Metaphorically (and literally, because you don’t want flu do you?) wash your hands. Return to the short story. Accept that the short story is not really a short story by any stretch of the imagination.
Pretend you were writing a novella all along.
Rewrite the not-so-short story from scratch. Despair when it keeps growing longer.
Rewrite the novella with an eye toward trimming some of that verdant verbiage. Growl in frustration when it somehow gains words.
Rewrite again. Keep it within the realm of believability re: word counts and re: novellas.
Finish something that could be called a draft.
Dig a hole in the backyard.
Yes, through the two feet of snow.
Bury the draft. Pat that snow back into place.
Practice saying: I never write novellas. What novella? I was home with my cats at 8.36pm on the 21st of January and you can’t prove a thing. I’ve never seen a novella in my life.
Wait for spring and see what grows.