I have the broad strokes already in my mind like what I want to happen but how most of that stuff plays out I make up as I go.
Creating your characters helps a lot too because you decide what their personality is going to be like and how they will add to/influence the story, and tbh that’s the most fun part for me.
When I have certain scenes in mind I want to happen I write them somewhere down and see at what point I can put it in the story.
I recently just published my first story and some of it is different to what I had initially in mind but that’s because I sat on it a long time changing and adding stuff.