I don't actually know the best way to write a screenplay. Fortunately, I don't have to know. They could fill BOOKS with how to write screenplays...and they have.
My favorite are those by Syd Field, and there are a few of them.
The blog title links to the one I am reading right now.
GGE writes all sorts of stuff, screenplays among them and I find this book to be, not only helpful, but inspirational.
So if you are thinking about writing and selling screenplays, check it out.
It can't guarantee you a sale or anything, but it will give you some good insights from someone who has seen lots of success