Hihi. These are just my thoughts, but remember that I don’t know much about Screenplays, so this may not be the best advice. I have, however, written a few books (never published), and I write plot for my current game, so I know a little.
Since its a movie, and not a TV show, you have to skip out on a few arcs, and you obviously can’t fit everything in. Make a list of everything you want to happen, and write the estimated time that you’ll think it will happen in. It should total up to be about two hours.
Not knowing a lot about your film, I’m going to make the assumption it’s Action and Adventure. Most movies from that genre are about an hour and a half, to about two and a half hours, so try to have your estimated time fit somewhere in there.
If it doesn’t total up to be that long, you’ll probably be fine adding a little bit more. If you’re writing for a company, it’s probably always best to have more in your script, rather than less.
On the other side, if its a bit longer, it might be best to keep it that way, for the aforementioned reason.
Now, when you finally do start writing, keep in mind that a page is about a minute, with just a few exceptions (most action scenes).
While I’ve never written a script, when writing this helps me. So, as I’m writing a scene, I imagine it like I’m watching a movie. For a screenplay, this would be character movement, angle, how the dialogue sounds, all of that. Then, you just have to find a way to put it on paper.
This is just some of my advice. Take it however you want.