Okay, here's the truth. There are WHOLE BOOKS that tell you how to write a screenplay.
Does it take an entire book? No.
I think what it takes is a screenplay.
The problem is, this doesn't work great in handout/blog format.
What I usually do in class is, I show my students my short film, Meaningful Touches. It's about seven minutes long, and the script is about 7 pages.
They watch the movie, and read the script, and in the end, they know how to write a screenplay. In theory.
In practice, they make mistakes. Which is understandable, because, as I said, there are whole books devoted to the subject. Format errors happen, and they happen all the time.
At a university level, professors will spend a whole semester teaching their students how to write a script.
When I teach, I don't have that kind of time. I have a week. Maybe two. And at the end, my students need to have a working functional screenplay to shoot.
So what I do is, I throw them in the water, hope for the best, and then show them where they're making errors, and when we're done they're writing screenplays almost completely correctly.
But how to teach this online?
First, check out this article on the subject of screenwriting style.
Read that article twice, then print it out and keep it on the desk next to you while you write. If you're not sure of something, format-wise, check the article.
Second, read a few screenplays while watching the movies they turned into. You can probably Google the name of your favorite movie and the word screenplay and find one you'd be interested in reading. For that matter, a lot of production houses make their award-nominated scripts available online, for free.
But if you need to find something that sparks your interest, I recommend checking out The Weekly Script. Why them in particular? Lots of scripts, lots of genres, lots of good movie choices.
And most importantly, they seem to be pretty strict about making sure they're using the correct/original formatting. Trust me, this is important.
If I were you, I'd print a script or two out. Just for reference. It'll be much easier to look at it that way, and when you first start writing scripts, you'll do that a lot.
At this point, I usually create a sample script on the blackboard and use it to show my students about 95% of the stuff they'll use when writing. The only problem is, screenplays don't usually show up correctly on blog sites.
Oh, there are ways to make it work, but it's a serious pain, and I could spend a whole day trying to get it right to give you a bad example of what a script should look like.
So read a script or two. And read the article I linked. And then remember this:
Sluglines look like so:
INT. BOB'S LIVING ROOM - DAY
That tells you where you are and what time of day it is.
Action looks like so:
BOB, a fat dude in small pants, picks up his remote.
TED walks into the room, holding a gun. Ted is just as fat as Bob.
(Did you see that? The name is only capped the first time it appears in a screenplay.)
That's dialogue. It should be short and to the point. If you really, really, really (and I mean really) need to explain how dialogue should be said, you use a parenthetical, like you see under Bob's name, but over his line.
And that's most of what you need to know. Now it's all just practice.
For what it's worth, you can get free screenwriting software here: CeltX
That'll make it a LOT easier to do your formatting, trust me.
So practice. Write your script. Then read a script or two (while watching a movie!) and re-read the article I linked, and go over your script and correct all those formatting errors you made.
There will be plenty.
And then you're on your way!