Develop the idea. A simple idea or concept may be very vivid and detailed in the mind of its creator, but the idea is useless unless it is developed. Take the concept and form a working story out of it. Brainstorm the rest of the story if it is not complete.
Write a short synopsis. A synopsis is a one-to-two page document that outlines the basic premise of the story. It should include major characters and the most important points of the plot. There is typically very little dialog in the synopsis. Anyone who reads the synopsis should have a very clear idea of what the film is about. When doing an initial pitch, this synopsis may be requested.
Write a detailed synopsis. This will serve as a skeleton for the script and help the writer when they work on the script. The detailed synopsis includes all the scenes in the film, some additional dialog and more details about the film. The detailed synopsis is usually around five to ten pages.
Learn how to write a script. The script is what sells; for first-time writers it is rare to simply sell an undeveloped idea unless it is very unique. The script is what will make money. A typical film script is 90 to 120 pages and formatted in a very specific way. Scripts that are not formatted correctly are regarded as amateur and often dismissed outright. Programs like Final Draft and Script Magic are used to help writers with formatting. Read a book on screenwriting, tutorials on script formatting or take a screenwriting course if you have no experience.
Write the script. Be dedicated and work on the script daily. The detailed synopsis will help. Read and re-read the script several times when it is done. Read it out loud and edit it. Have the script edited professionally or by a friend who has the right skill set.
Pitch the script. This is by far the hardest part. It is nearly impossible to walk into Warner Brothers and go "want to buy my script?" The script will likely be purchased by independent producers or production companies. Find a list of production companies by visiting film directory sites like IMDb or by researching the production companies credited on your favorite films. Submit the script with a cover letter to the production companies. The cover letter should give a sneak preview of the script and outline any relevant experience you may have or details about the project. Many established production companies do not read unsolicited material.
Get an agent. An agent or manager will open many doors and help get your project out there. They have contacts in the industry and can pitch the script for you. Getting an agent is very difficult unless you have a significant body of work.
Network. Go to film-related events, venues where film producers hang out. Go out and be seen. Tell everyone you know that you are shopping your script around.
Prepare for rejection. It is very difficult to sell a screenplay and it is not uncommon to see a writer with over ten completed screenplays still trying to sell just one after many years. Tenacity, a thick skin and patience will be required.