In terms of developing particular screenwriting skills, well, you need to understand that screenwriting is a very precise form of writing, with very particular conventions. You really do need to understand conventional story structure (as applied to film), character arcs, screenplay format, language conventions and dozens of other things, that only apply to screenplays. There are more DO's and DON'Ts in screenwriting than in any other form I've ever worked in... and they're all in place for pretty good reasons, and ignored at people's peril.
I don't understand what that means. I would decide all that subconsciously. You mean like when the action takes forever? That's a don't. In particular what do you mean by "precise?"
Film scripts have to deliver information to a huge team of people, all looking for different things. In the first place, the script has to tell its story in such a way that a producer, director and good actors can all visualize the end product... and, at the same time all have confidence the end film will create enough sales in order to justify the expense of making it. Now, the producer reads the script and wants to know what the poster and the trailer will look like, because that's how she'll sell the end product, the director wants to know how it will be shot... and the actor wants to know her character's journey and whether the role is going to make her look good.
Your script has to do all of those things... sell the story, make a clear vision for the director (without directing the film for her) and at the same time create compelling dramatic roles for the actors.
On top of that, producers and directors judge the duration of a film by the number of pages... working on the formula: 1 page equals 1 minute. That formula only works if you format your script the same way as everyone else in the industry... and, also if you use language in the same way.
And, as if that wasn't enough, the Art Director is using the script to figure out the look of the movie, the Location's Manager is figuring out where and how to get each scene's location... along with several hundred other people whose job revolves around interpreting the script... and, all of them wants just the right amount of information: enough to inspire them, but not so much that you're doing their job for them.
Now, because you're servicing the needs of all these people, screenwriters have developed a style of writing which is unique to film making... a lot of which Film Jumper talked about.
So, for instance, the most natural thing in the world when writing a film sequence is to write:
We see Jimbo, a tough eleven year old, climbing the tree outside the old Patterson House. He has a duffle bag on a rope, tied to his ankle.
When, actually, it should be:
EXT: THE OLD PATTERSON HOUSE (1979): NIGHT
From the boarded up windows, to the overgrown grass and decaying porch furniture, everything about the old Patterson House tells you it's the spookiest old house in this small Mid-Western town. Practically growing through the house, a KNARLED OLD PINE TREE.
JIMBO (11), a tough little tear-away in an "A Team" T Shirt, clambers up the branches of the Pine Tree, a DUFFLE BAG suspended from his ankle on a SHORT ROPE.
As to the other stuff:
Conventional Story Structure:
Either three, four or nine Act, depending on what works best for you; but, with an understanding that anything write is going to be read as if it was constructed to a three act model. The basics of three act story structure has been outlined by both Syd Field and Robert McKee.
Character Arcs:
The journey the character goes through during the movie... normally the plot for the film is the protagonist's character arc... and in some films that's all there is. However, if you're writing drama, it helps get a better cast if all the characters have some kind of arc.
Screenplay Format:
The way a screenplay is laid out... some of which you get just by using screenwriting software like Final Draft (expensive) or CeltX (free).
But beyond that, an understanding of when you put a character's name in CAPS, what a Lugline is and why you almost never use the expression "We see"
Language conventions:
In action description it's important to hit three points: keep it visual; active voice, not passive (so "runs" rather than "running"); and, finally, short sentences... avoiding complex compound sentences where possible. LOL
In many respect Hemmingway is a perfect model for language usage.
Hope some of this helps.