Some thoughts for writing quick fiction:
Start with a few brainstorming sessions and take solid notes.
Write an outline for your story and then write out the beats of each chapter/scene.
Take each chapter one at a time or write them all in a rush.
Comb through chapters for errors such as misused character names/pronouns/other details.
Stay flexible to your own ideas, as they may change throughout the writing of each chapter.
Have fun and be kind to your mind.