(1) I've been SUPER busy on a script and thus less active in...everything. BUT I've been writing a couple of sample pages to use as a format tutorial. They're not ready, but I'll have them
(2) This thread and language use:
This should be your default when typing descriptive text of any kind (IE anything that's not dialog), but it's not really a hard-and-fast rule. Screenwriting is sort of its own language in the sense that you're conveying certain technical information ("CUT TO") as well as commands ("she blinks") AND that ever-important rule "don't be boring." There's a lot of cheating with sentence fragments because it keeps your lines really concise ("He locks, loads, and leaves" as opposed to "He picks up the pistol, opens its cylinder, and gently inserts six rounds.").
Punctuation is really important. You need to get a sense of how each mark is 'read' and try to use it accordingly. Again, there's a lot of cheating in screenwriting...this isn't a grammar class, it's a work of art, so you use punctuation as a form of expression. As long as your punctuation can be understood, it's OK to do a few strange things.
Don't make simple mistakes...a few typos will always be forgiven, and some very famous screenwriters didn't finish high school (Tarantino's scripts are full of 6th grade level errors), but your work needs to look like it's written by someone who is deeply involved in the written word. It's good to look like you know what you're doing.
Dialog is 100% character conscious...one person might speak proper English - they'd say "I don't want to play marbles," but the next says "Don' wanna mahhble, man" the next says "FCK your marbles!" When you're writing dialog, the only rule is "be understood" - if your actor or your reader can't interpret what the line means or what it's supposed to sound like, it's no good. Other than that, anything goes, and I mean ANYTHING. If you're not having fun writing dialog, no one's having fun reading it.
Slang in the descriptive text is a little more case-specific...you might use it to indicate that a character is hiding something by inserting "Liar liar" after one line and "Pants on fire" after the next. You might use it to spice something up, "He snorts a line....it's the sh*t that killed Elvis!" Or maybe a pop-culture reference is going to help sell your description, "He's wearing Gandalf's beard."
The main thing you want to avoid is looking like your script is written by Beavis and Butthead. "Then this dude goes up to the other guy, and he's all like:" Never do that....it just makes you look like you're not serious.
Writing is all about which words take up the right amount of space on the page, or in the mouth, or on the brain...whether the words you've selected express - to the best degree words can - the pace, atmosphere, and motion of a real movie. To the extent that slang aides you in that, it is a worthy ally. To the extent which it detracts, it is your mortal foe.