My system, and I know from hearing it from lots of other writers, is to just write. No planning, or not much of one.
But even with an outline, a lot of fluff is going to go into the manuscript that is unnecessary, boring, unclear, or for some other reason needs a good edit.
I learned about cutting ten percent when doing lots of short stories. I like the idea because it’s a straightforward way to edit. And it works.
First, you finish the piece and type “the end” at the end. Then you read through it and cut out every single word that isn’t needed.
They lurk around as modifiers where a stronger noun or verb would work better. They hang out at the end of sentences in trailing prepositional phrases. Sometimes they’re completely unnecessary sentences. Or even paragraphs.
Sometimes you just need to change the phrasing so it’s cleaner and clearer. I find this true when listing action taken. I might start with one action and tack on a phrase like “after she did such and so.” Such and so would have been better at the beginning of the sentence. And might help eliminate a few words such as “after she did.”
I admit, this becomes harder to do the more you write because you begin to edit yourself as you go. But this is a good thing. You will write faster, have to edit less, and have a much better story to show for it with less effort.
So, take a short story, old or new, or a chapter from a novel you’ve written or are writing. Write down the word count, go at it, and then see what the count is after your first pass-through. Not ten percent? You aren’t working hard enough. Go through it again. It’s taken me, sometimes, three or four passes to hit the ten percent mark. But when I do, the story or chapter is always better. Every time.