I'm aspiring screenwriter. The way I do is as follows...
- First you come up with a concept, the basic idea and theme of the story. How do I come up with an idea? There are many ways. There is nothing to solve when there are no problem. So first you see the problems around you, around society, you see the issues in the society, you think about things thatyouI care about. Sometime you don't start with a problem, but you innovate. You just imagine things, make things, how about if things happen like this, in a different way, you ask yourself. Sometimes, when you watch a movie, you'll suddenly get some new idea, totally different or partially related to concept the movie you're watching but with a different thought or way. You'll get ideas when you read news, when you go out, when you watch some uncommon incidents, etc.
- So now you have an idea. How do you tell this as a story? You need a plot, you need something like a begging, middle and end, it's called 3 act structure. All movies not 3 act structure. So it can be in a different structure also. But you need a plot of one or two pages.
- Now you have a plot. A plot doesn't include all the characters. Initially, when write, you don't care about the characters, just think like how to tell the story. How to start with, what is the problem, how it get solved, etc. This is called plot based/driven story. There's another way, it's called character based story. I'll explain that at the end.
- So when you have a draft version of the plot, you start developing the plot by adding few main characters and clearly specifying the problems, issues, goals, and how it solved, etc. And you'll give get some clear structure of your entire story.
- Now you have a better plot. You know some of the main characters of your story. Now you have to get into details. Who is the main character, why he is even in the story? What he is doing? Why he becomes a lead character in the story? What's his job? Where he lives and with whom? What is his problem? What are his strengths? What are his characteristics? Who is the antagonist? The antagonist is not always a person. How does the protagonist pull into the problem? How does he solve? This ways you write a draft version. So this draft version includes scenes, locations, dialogs, etc. When you write the plot, there is no formatting rules, you just write whatever the way you like, but when you start writing the script, you actually write in screenplay format. Use software like Celtx, Final Draft. Celtx has a free version, it's great software for beginners.
- Now you'll polish the script. Looks at the problems in your script. Is there something totally unbelievable? Is there any logical problem? Is the dialogs natural? Polish all the dialogs, makes it more natural. Is there any scenes which are unnecessary? Do you need to inject any new scenes or dialogues, in order to create conflicts, create emotions, or interest for the audience. How to make your script unique? What elements you have in script that none or most of the movies didn't have? So like this you write 3 to 5 times of revisions of your scripts.
- Next, you tell the story to others. Make them to read. Get feedback. Validate them. Analyze it. Make necessary changes. So again, you work for months to improve it.
As I mentioned earlier, there is another way called "character based/driven story". In this you mainly start with an interesting character, his problems, is ways of doing things, etc. So you don't create the plot mainly based on your idea, but you take the idea and the character, and you mainly follow your character to create the plot.
If you have a chance, you surround with people having similar interests. You brainstorm your idea, get feedback, and further develop it. Sometimes, two people write the story together.
Many times you lock yourself in a dark room, and imagine the story, visualize the characters, locations, scene, etc.
When you start writing your story, mostly you'll know what kind/genre you are writing, so you'll watch similar genre movies and read same genre scripts. You'll get an idea from that. You'll also go to library, read only about the story you are writing, you do your research. For example, if you are writing a story about a farmer, you need to read about farmers, their life, etc.
You can also storyboard your thoughts, scenes, locations, actions, etc. You can do it by yourself, or take someone help, or use some softwares.
There are a lot more things. You can also read some screenwriting teaching books. You can submit your script to screenwrting competition and get feedback.