How to
Note: I wrote this many years ago and only now decided to post it. I don't know how useful it will be, but I have been asked before how I write so here it is.
The first thing you need is an idea. Pick one that you really like, as it is the basis of all that comes next, but keep in mind that you can (and will) change it as you go along.
This idea should be very broad, think two or three short lines of text, no more than that. It can come from a picture you saw, a movie, a dream you had, or something you read. A fantasy or anything you want, so long as you can work with it.
Now that you have it, write it down.
Once you have your idea, you can start expanding it. I mostly do this in my head, as it can take a few days of mulling it over for it to develop, so take your time. Again, we are being very broad here. Think about it, where do you see with it? Can you expand it? Do you need to change it? This is the time when you decide if you keep with it, or if you scrap it.
After that, if you were able to expend it, it's time to write down the sketch or a script. Do this as fast as you can. Don't stop to correct mistakes, or to give it any structure. The idea of this is to give you something very rough that you can follow later on.
I generally do this in single lines, like ‘first this, then this, after that this, describe that, then add this’. If you thought of a particular line of dialogue, or a reaction from one of the characters, this is the place to write it down. It is fast and messy, but it's the idea. If you stop, you will often get stuck.
This one will be longer than the first idea, but it won't be too long. For a 20 page story (once finished) this is usually just 2 or 3 pages of short sentences.
Once this is done, read it again, see how it looks. Maybe you will make a few changes here to improve the flow, or you might remove something because you realize that it just doesn't fit in.
You might also want to think about who your characters are and what do they look like. I usually describe their age, and appearance, along with what they are wearing (if it is important to the plot). How they act (shy, extrovert, crazy, etc) should be on the script itself, not here, so this is optional.
Finally, it's time to write the first draft. Like before, I recommend to write fast and without stopping. I usually get stuck every time I stop, and it takes me a few minutes to continue. The idea of this is to start with the finished story. Here you are going to add everything. You will follow the sketch, but feel free to deviate as you go along, as it is just a rough guide and you should follow the flow of the story.
Add dialogue, feelings, situations, everything you thought of. Try to pay attention to the details, but not to errors as you type, just keep going.
This will be the longest step and it can take several days or maybe just a few hours.
You will probably decide to add or remove things as you go along, and that is fine, but don't bog down on the details, just try to get it out on the screen. If you need an specific detail (like the name of something you just can't remember) that you don't know, don't stop to look for it, just write it IN CAPITAL LETTERS so you can come back later to change it.
Once you are done, it's time to correct all those mistakes. Here the spelling tool is you friend. Correct all of them to make the next step easier.
Once that is done, it is time to read it all. You will find mistakes, incorrect words and structures that make no sense. You will also add and remove entire paragraphs as you think it over. You might add or remove characters, or improve on the dialogue (the hardest part, at least for me).
Once you are done, do it again, this time it will be easier and you will be able to pick up on things that are contradictory, or details that are missing because you have it all in your head.
Once it's done… I usually give it a day or two and then read it one last time. All above this, I can do in a day, or a few days continuously, but a few days away from it can give you a better perspective.
And then you are done. The only thing missing is a title. You probably thought of one along the way but sometimes it's really hard. Think about it, and you will get one.