Writing Your Movie
Take a blank piece of paper and write down numbers from 0 to 100 in a row. Each one of those numbers will be a scene. Even if each one of those scenes is a minute long we got a movie that's 1h 40min long! Think of each scene as a gear in a clock, it has to fit, make sense, and push the story forward. Take your time, have a coffee, it will come to you.
I think this is the perfect way to start writing a script as it gives you a visual representation of what you have in front of you and a sense of direction where you going. Some writers use index cards, I don't like that much and find myself using this technique most often instead.
Once you have your 100 scenes, start writing more in-depth. I assume you know how to format a movie script, if not research it on YouTube it's very easy. Few tips: keep it in a few locations, fewer actors better, no aliens, no explosions. Remember less is more, and it shows.