Okay, so once you have your initial idea for a story about a character, you have to make sure you know how the story is going to flow

What I do, is I write notes under the title about what specific bits of dialogue I want to include, or how an event should happen

The notes don't have to be incredibly detailed - just enough so that you can understand what you've put

By doing this, it allows you to generate more ideas for either this story, or maybe even a completely different story

For example, here is some of my planning for a story in my Bucky book that has been sitting there for literally like 78 years-
(Rip dark mode users)

See? These little notes are simple, and they can be added in even if you're in the middle of properly writing it- you may have thought of a new idea for the story- that's great, whack that bad boy in there

Just make sure that you delete the notes either while you're writing and you get to that part, or when you've finished completely