What you're experiencing is called the "incubation" of ideas, and it goes
on for a long time before creativity begins. The longer you mull over ideas and plans, the better your writing is likely to be, unless you just happen to be another Ernest Hemingway or Virginia Wolfe.

Ideas have to mature. When you're ready, then comes the writing, re-evaluation, editing, and refining. Oftentimes, the complete story will just suddenly appear at a serendipitous moment.

I know of some who put their ideas on tape and later put it on paper, organizing,etc.

And, oh, man, nothing's worse than sitting in a course in college knowing
that you need to write a full explanation of something and writer's block hits you suddenly.

Much work goes on in the head for a long time often in preparation for
creating something.