Here is a fantastic resource for you writers out there: FREE links to the screenplays of every Oscar contender.
Screenwriting is a completely unique discipline. It feels as if you have to rewire your brain to think in script form with all its formatting peculiarities. If I were trying to explain or teach someone how to do it, simple instructions would be nearly impossible on the more subjective elements. How much detail should I give in my action blocks? Should I include any camera/framing directions? Personally I've found nothing is better for finding the sweet spot and getting your mind into the creative rhythm of a script than reading an abundance of screenplays by the writers I enjoy the most or from a specific film that I loved; to see how it translated from page to screen.