Full details are in the Manual (Chapter 21), but basically…
First of all, choose your type of screenplay / script from the Format > Screenwriting menu.
Then make sure the document you want to write is in Screenwriting Mode — toggle cmd-8 till it is. You can tell whether it’s in Standard Text mode or Screenwriting mode by the bar at the bottom. In Screenwriting mode, a drop down box appears in the right hand corner, saying something like ‘General Text’. The setting is per document in the Binder, not per Project, so you can mix and match.
Click on that box (or press cmd-shift-y) and you’ll see all the standard elements for that type of script, including Character, Dialogue, etc, with a short cut key.
So start off with Scene Heading by clicking on the drop down list and choosing Scene Heading (or cmd-shift-y s). Type your Scene Heading, but don’t press enter yet. You’ll see that the text is automatically formatted in Scene Heading format.
Look at the bottom of the screen and you’ll see some instructions have appeared:
[Tab] Action, [Enter] Action Scene Heading.
Press either Tab or Enter and you’ll see the next line has changed to the Action element as promised. Type something action-packed… and then press ether Tab or Enter again. The bottom of the screen has changed to:
[Tab] Character, [Enter] Action | Action
In other words, Scrivener knows that after an Action element, you’ll probably want either a Character or more Action. Press Tab and insert the Character name. You’ll see it’s centred and in capitals. Now the choice below is
[Tab] Parenthentical, [Enter] Dialogue | Character
And so on through the script.
In other words, most of the time, after you’ve started the scene off, Scrivener guesses what you’ll want next and you only have to press Tab or Enter to get the appropriately properly formatted elements.
If by any chance you’re not offered the element you want, just cmd-shift-y and choose the correct one.
If at any time you find that this isn’t working, it’s probably because you’ve come out of Screenwriting mode, so press cmd-8 to get back in it…
There’s other things Screenwriting mode can do (automatically complete character names for example) so do read the manual, but hopefully the above is enough to get you going.