The concept of a film is in simple terms, what the movie is going to be about. For some people, this may be the hardest part in the creative process as it can make or break a film, but some may already have an idea. From here, the concept should be developed, stretched out and planned.
Usually a, or a team of writers will sit down and plot out the story, highlighting major key points and refining some concepts and coming up with a timeline of events.
And now the actual writing process begins. The screenplay is the format used for writing films, as it provides a good amount of deal to help guide the filmmaker in creating the film. It is similar to a Blueprint, but it isn't as exact as it allows room for other creative decisions.
After the screenwriter writes the screenplay, they will generally rewrite it, removing unnecessary sections and adding scenes to the film flow and be better understood. The first pass for rewriting will fix grammar and other small errors. It is estimated that 80% of writing time will be spent on rewriting.
After the numerous amount of drafts and time spent rewriting, you should now have a pretty decent script. So what comes next? Pitching. Pitching is where you take your script, and throw it at a bunch of people, in the hopes that someone will pick it up, pay your money and make it. That's the basic idea, and how you go about that is obviously the deciding factor.