"Cinema is a mosaic made of time." - Andrei Tarkovsky
Screenplays are a strange form of writing. There is a certain utility to it that makes simply reading through the text insufficient to judge its overall worth.
Unlike with plays, most screenplays you read explain motions, aka use action lines, with almost as much specificity as they prescribe dialogue. Shakespeare is infamous for his vagueness in character's actions-
(What did Banquo tell Fleance to take? It's driving me crazy! Was it a dagger?? Is that the dagger Macbeth sees??? Then why is that dagger intangible if it's real???? WHAT'S GOING ON, BILLIAM?!?!?! *Ahem* sorry.)
-because there is little need for a playwright to specify actions that most people in a crowded theater will not be able to see clearly anyway.
What this amounts to is that screenplays have a much greater need to exert control over several aspects of the story (setting, line reading, actions, etc.) while maintaining a comparable length and structure to a traditional play.
As a result, screenplays are a compact, fairly regimented and rule-based form of creative writing. Unlike, say, a villanelle, which is a highly technical form of poetry, screenplays have the added pressure of needing to tell a whole story and demonstrating each aspect of that story visually. (There is no, "and then the king and queen lived happily ever after" in film: the 'happily ever after' needs to be painted in minute detail).
As a script consultant, I have developed an eye for screenwriters that struggle with the strict confines of their medium. Writers are creative people after all: it is in our nature to follow our art down interesting, unexplored paths. A novelist gets to take three pages out of their story to meditate on the beauty of a landscape or the nuances of a relationship, stuck in time until they decide time gets to continue again.
This freedom is not advisable in screenwriting, because most of the incorporeal aspects to your story will only be visible if they demonstrated by or alluded to by characters and plot.
Consider the film Perfect Blue by Satoshi Kon: the incredibly detailed diagnosis of the mental illness Mima Kirigoe is suffering from is depicted through very precisely controlled visuals and pointed actions. Her journey is made perfectly clear while generating empathy, without pages and pages of Mimi's own self-reflection stated in words. Had said pages been used instead, they would either be in action lines, and therefore practically invisible, or stated in dialogue, ruining the plot and pace of the film.
And so, writing a screenplay is about stripping a story down to its bear essentials and constructing it in as much detail as possible.
Effectively, it's about saying a little with a lot.
That doesn't mean that film allows no room for exploration and nuance. It simply means that if those things aren't working to support the core of the piece itself, they risk wasting the limited amount of attention and time a reader/viewer generally allots for a screenplay/film, not to mention straining the very structure and pace of the piece unnecessarily.
(That said, just about anything can work to support the core of a story: a joke might enhance the desired atmosphere; an abstract dream sequence might push the main character toward an important revelation; an interesting backstory might lay clues for the main mystery. But a writer has to be honest with themself when they judge whether or not an aspect of their screenplay is relevant, which is often the hard part.)
This is why starting your screenplay writing process with a logline is a solid place to begin.
Organizing a logline is the best thing you can possibly do to show that you are in possession of a screenplay-ready story. Remember Aristotle’s model: sure, it's vague, but it’ll get you out of the weeds.
There is a complication and an unraveling. You’re logline will demonstrate the conflict and point toward what the unravelling ought to be.
Think of a logline as an equation. Consider what you are beginning with, what you are adding, and what those two elements will equal once the story as unraveled.
Or, more simply, "if what, then what?" or, "what if this, but then this?" Answering this question will give you a comprehensive logline upon which a unified story can be built.
When creating a logline, first figure out what you already have. If you have characters, get it all out on paper. A magic system, a history, a moral, an allegory. Whatever you have, get it out and take stock. If you don’t know what you have, how can you know what you need?
(And if you don't have anything yet, it’s time to get your inspiration going. Watch movies and TV and play video games and read plays and listen to podcasts and read books and just CONSUME. There is nothing new under the sign, you’re merely looking for an attractive angle from which to catch the light.)
Then, consider everything you have and ask yourself one question: what's the story here?
Having a character means nothing if you don’t have a sense of what the characters are going to do. Similarly, having an idea for an epic climactic scene means nothing without a sense of the action that makes that climactic scene so epic. An incredibly rich history or magic system can be a great centerpiece for a screenplay, but only if something happens because of it.
Story is about unfreezing your ideas: removing them from their timeless pocket dimension into a space where there is movement and life.
So, that's how to start writing your screenplay. Get all your elements assembled, and hit play.