Hundreds of full-length books have been written answering the question of "how do I write a script?"
To say that I could condense everything down into one blog post would be a lie. But, I can provide you a rough map to chart out the steps of writing a screenplay, with plenty of avenues and offshoots to explore at each stage.
You don't have to follow these steps in order. In fact, I recommend you don't, and that you follow these steps in the order that feels best to you. I do, however, believe that you will have to follow every step (at least once) to write a screenplay.
This is all to say that this post doesn't provide the way to write a screenplay, but it does provide a way to write a screenplay. Your way will be your own.
Before you can write a screenplay, you have to know how screenplays are written. Luckily, they follow relatively simple guidelines. Screenplays consist of two things: what the audience can see, and what the audience can hear. (You can check out this great article from Writers Store detailing what those pages look like, or this one from ScreenCraft that focuses on television scripts.)
There are exceptions to those rules, but I wouldn't worry about them especially if you're just starting. You are creating a written blueprint for a movie or tv show, which by nature are audiovisual mediums. So keep your screenplay to audiovisual writing.
The screenplay format has a bunch of technicalities that have developed over years to make it easy to track timing, identify actors, props, create a clear read, etc. In the 21st century, we don't have to worry too much about the details behind the formatting, because we have screenplay software that does it for us. Our software of choice is Arc Studio Pro, but any modern software should make it easy to format. (If it isn't easy, find a new one!)
Open up a screenwriting software (you can try Arc Studio Pro for free) and just play around with the formatting, making sure that you keep your writing only to things that can be seen or heard.
Ideas are loose and scattered, while a premise indicates a story. So before you start writing too much, make sure that your idea is strong enough to be turned into a premise.
A toy that comes to life is an idea. A toy that comes to life and is threatened by a newer, shinier toy is a premise.
One way to think about a premise is that it includes an obstacle or a challenge, whereas an idea likely doesn't. When you have a premise, you're on to something that can be turned into a story.
Keep a notebook and write down all the ideas in and around your screenplay. Note which ones are ideas, and which ones are premises. Explore what can be done to turn your ideas into premises.
The theme of your screenplay is the thing that connects it to the audience. It might be called "the bridge of relatability." The specifics of your story make it unique, but the theme of your story makes it universal.
A theme is more than just an idea or word. It is your opinion around that idea or word. "Family" is not a theme, but "Family is the most important thing in the world" is.
The main character of your screenplay will likely, by the end, reflect the theme. They will conduct themselves in a way that bears out the prevailing theme of your story.
You can see how characters express theme in this great video essay on Jurassic Park from Lessons From The Screenplay:
Write a few statements that could be a theme for your story. Which one fits the best? As you continue to rewrite, revisit this exercise and make sure it still fits, as your story may change. Identifying the sharpest theme you're exploring will point to opportunities to explore it further.
Most stories are driven by the main character. The screenplay should not be something that happens to them, but something they make happen. To do that, you must ensure that your main character has a want.
Definition: Want. This want is what drives the plot. It is a goal, and they take steps to get it. The steps that the protagonist takes to achieve this goal are the things that reveal their character.
Do they cheat, lie and steal? Or do they hustle, strive, and overcome?
Filling out your protagonist with the specific ways in which they operate within the world is what makes them unique. (And the specific plot points they encounter that take unique solutions to overcome shape them as well, but more on that later.)
A good main character will likely also have a need.
Definition: Need. This is something they may not be aware of and is likely a lesson that they must learn.
They will likely not achieve their want if they don't learn the lesson that is their need.
Contradicting wants and needs is what can help make the main character complex and compelling.
For example, in The Wizard of Oz, Dorothy wants out of Kansas. She wants to be in a more exciting and fun place. By the end of the film, however, she realizes that she needs to be surrounded by her family and those she loves.
At the beginning of Star Wars Episode IV, Luke wants adventure. By the end of the story, he has realized that what he needs is trust (in himself, others, and also the force).
Think about your main character. Do they have a clear goal? And do they have clear room for growth? Making sure these two things are intrinsically connected will ensure a compelling character arc.
Arguably, the structure is the most technically important part of a screenplay. There are many different models for screenplay structure, just as there are many different architectural blueprints for different types of buildings.
When first starting, I recommend not trying to differentiate between which of these models is better than the other, but simply identify which one seems the easiest for you to follow and understand.
As you're shaping your story, these structures will help you identify where the big turns are, and help you make those shifts in your story that take it from beginning to end.
The structure can help you see how scenes fit into sequences, and how sequences fit into acts.
You'll also likely encounter the words beats, turns, pinch points, plot points, and all other sorts of vocabulary.
You don't have to memorize these to write a screenplay, and there is no real agreement on what exactly they all mean.
Make sure your story can be broken down into distinct parts. Do those parts naturally fit into one of the many models of screenplay structure? If not, then your story may be like a run-on sentence, just going and going, but not really changing, and saying the same thing over, and over and over, again, and again, while your audience is waiting for a shift to get to the next thing.
Identify and/or create these shifts, creating a structure for your screenplay.
Now that you have a good view of your structure, it's time to get a little messy with it. Following the models to a tee will have your story end up as the median average of all movies or shows.
Vary it up by extending certain sequences or truncating others. But don't do that arbitrarily.
This is where you can forget about structure for a bit and have fun with the story and characters.
Try to get in the mindset of your characters, and "let them make their own choices" even if it seems to go against the structure or plot.
You'll likely discover unique ways to get to where you're going and now your story is starting to be more and more "you."
Whether it's an outline, treatment, or actual pages, abandon any plans and see if your characters can "break the plot." Explore what happens if they do everything to avoid what you need them to do to move the story forward. This will present you with a) a true-to-life character, and b) options for obstacles in their way.
My advice for scene writing is that before writing one, really make sure you know what each character in the scene wants. Try to identify it, focus on it, then turn everything off and just write actions that are them trying to get that thing (or avoid that thing if that's their goal.)
Scene writing is hard, and most early pages do not seem like real life, and they don't seem like the movies either. That's okay. You will rewrite them later.
But right now you just have to trust that all that work on theme and structure and character is rolling around in your head while you're focusing on writing a character trying to get what they want in this one moment in your story.
Write multiple versions of the same scene. Write one, and the next day, try to write the same scene again from scratch. Don't look at the previous day's work. You'll be amazed at what sticks in your memory, and what doesn't and it can be a great way to identify what is crucial, and what is not.
If you ever get stuck on what a character will do or say next, on a notepad next to your keyboard just write a list of all the things they won't do or say. It can be a great way to discover the next beat in the scene.
If I had to rank these steps in order of importance, this would be at the top and everything else would be tied for last. Figuring out how to keep your engines going on good days and bad days is crucial, and being able to show up consistently is the key to good writing.
Forming good writing habits means looking at how you feel when you create and structure your routines around writing in a way that supports you. Some people do small daily sessions, while others have one or two days a week that they completely dedicate to writing.
Assessing yourself and your needs is the key to discovering the best practices.
Feel free to get lost for a bit in optimization techniques and productivity hacks, but don't spend too much time there. Listen to your mind and your body, and balance your time accordingly. Celebrate the time you spend writing and not the quality of the output. That will come if you just keep writing.
Track your writing habits and track your mood and feelings. See if you can spot any correlations, and then make small adjustments until you find a schedule, rhythm, and routine that works for you. If after a while that starts to feel stale, mix it up, and try something new, but always listen to your gut (so long as it's not the part of the gut that wants to procrastinate.)
A screenplay isn't designed to be kept in a folder on your computer. You have to get it out into the world, and that means getting it to read.
Starting, this likely means reads from peers, which can be found in local or online communities. You may even be able to join or start a writing group that meets weekly to trade pages and feedback.
When you get feedback, the key is to stay open. Even if a reader's reaction or opinion seems way off, you can always look to learn from it. And a reader's solution may not be the best either, but seeking to understand why that's what they're pitching can help you see the best solution for your story.
Find a group of readers for your work, whether it's a local or online community. Get your screenplay out to a few people, and be open to their reactions and thoughts.
It's cliche because it's true, but "writing is rewriting."
Rewrites are where good stories become great.
This is where you take the feedback you've received and make changes. Some may be small, but others may be big. Don't let the size of changes that make your story better keep you from making them.
I've heard rewrites sometimes described as breaking a bone that needs to be reset. It can hurt and feels like you're moving backward, but it's the only way to go forward in the long run.
One thing that's worth keeping in mind is identifying whether issues are "script problems" or "story problems." To me, script problems are places where the intent was right on, but something in the writing wasn't getting through to the reader. It just needs to be clarified. Story problems are choices that are made that fundamentally do not work with what you're trying to accomplish in your story. (Another way to think about these may be technical choices vs artistic choices.)
Identifying which is which in your script is half the battle in solving them.
Create a list of things to accomplish in your rewrite and seek to address each one individually. Don't rewrite page by page, but rather problem by problem.
This is a very simplified roadmap of how to write one, and no matter how much you read about it, it's always easier said than done. But these steps outline the broad steps of what you need to write a screenplay.
And if you followed them all, and you did it, and you haven't lost your mind, then go ahead and start the next one. It's gotta be easier than that first one, right?