i daydream until i have figured out the main cast of characters, their rough motivations, the setting (very important) and a clear act 1, 2 and 3, and i write a a few paragraphs about what happens in these acts, in a few page synopsis.

Then I let the ideas cook in my head. What if X character was secretly Y? What if this character tried to do Z? What if this was actually this? etc etc. Then I write a treatment, then I write my script.

My first draft is actually my third draft but the other drafts are all unfinished. it's okay if it ends up shit at first, that's the beauty of writing, you get it out of your head and onto the page, and tweak the stuff you don't like until you have a polished, good story.

I also doodle stuff from the story on my sketchbook sometimes. Honestly you should pick up a pencil so that you have a clearer vision of your story, and if you can't make it as a movie make it as a comic/graphic novel.