Before writing the actual screenplay, become extremely familiar with standard format, including length, font, indentation and act structure. There is a lot of software available tailored specifically to screenwriting that automatically formats to the industry standard, and this can be much more convenient than using a simple word processor. While there are exceptions, script readers for production companies and studios review many submissions and even a slight deviation from accepted format may make your work appear unprofessional, which will often cause it to not be read. Ian Briggs of StoryInsight.com recommends researching what is expected from the industry, including details as small as the type of paper used . Different genres often have varying acceptable lengths, and it is important that you stay within this range, especially if your name is not yet known within the business. The Writers Guild of America provides useful formatting resources on its website.
Gain exposure to as much screen writing information as possible, in order to familiarize yourself with what kind of material Hollywood is looking for. There are many books on the subject, and even formal classes that teach the craft. Reading scripts of successful movies will be a great help in writing your own masterpiece, but make sure you are reviewing actual screenplays and not just transcripts of the movie as it was actually made. Scripts endure significant change during production, so you need to know what will be successful in creating the screenplay itself, not how the script looked on production day.
Structure your story in detail, including conflicts, resolutions and other core cinematic tools. Most screenwriters will write a treatment of their screenplay---essentially a short summary of the story usually written in prose. Treatments can vary in length and style, but generally should be limited to no more than a few pages. StormforcePictures.com describes a treatment as an outline of the screenplay, from beginning to end, and a valuable way to develop story structure.
Start writing. While a first draft offers less restriction than the final product, attempt to stay within time limits and not stray from the treatment unless absolutely necessary. A commonly accepted rule is that a page of your screenplay should translate to about a minute of screen time, though this can vary based on how dialogue-heavy any given page is. When writing dialogue, read it aloud, as what appears on paper can often sound much different when actually spoken.
Revise your draft at least once before presenting it for feedback from others. Screenplays are long, and almost all writers will have at least a few typos or grammatical errors to correct, aside from the usual revisions to scene sequencing and the actual story. Jennifer Arzt of ScriptFrenzy.org recommends taking some time off between completing the first draft and the first revision, so the material can be approached as freshly as possible.
Find others to read your script and give you constructive criticism, preferably someone with experience in the film business or critiquing dramatic work. While many writers are hesitant to hear criticism about their work, it should not be taken personally, and good writers use this criticism to improve. No writer consistently sells a first draft, and it would be a grave error to ignore this important step. Revise as much as possible; the vast majority of successful writers create at least two drafts and you will almost certainly not be the exception.
Register your screenplay with the Writers Guild of America. Although this does not offer as much legal protection as a copyright, it will do wonders to prevent others from using what you wrote without giving you credit or compensation. Hopefully, many eyes will read your script, making it extremely important that you legally protect your work.
Get your screenplay read. This can be done many ways: make unsolicited submissions to studios, find an agent or send out scripts to anyone you think can help. Getting read is perhaps the most difficult part of the process, which makes it even more important that you have a screenplay that is not only good, but exceptional. Getting someone with power to read the script is challenging and valuable, so no chances can be taken that the material itself is not up to Hollywood's standards.