Organize your ideas. A typical screenplay book should encompass about three major sections, each with its own set of chapters. The sections will roughly cover "screenplay format," "story development," and "marketing your script." You can title the sections with these titles or come up with something similar. Use a word processor to draft your book. You can save as PDF for electronic distribution later.
Outline each section with chapter ideas. The formatting section may include chapters on page setup, element formatting, how to introduce new characters, and other subjects that relate to the specifics of formatting in a script. The story development will include chapters on setting, character development, how to develop a three-act structure, text and subtext as well as the particulars of developing the story itself. The final section will include chapters on writing a treatment, writing a log line, how to pitch a script to agents and producers and other ideas for marketing your work.
Include a chapter that discusses screenwriting software in detail. There are several programs used to write screenplays, including Final Draft and Screenwriter, which are industry standards. Discuss alternative screenwriting programs as well. These include free alternatives such as Celtx and web-based programs such as Scripped (see resources). While most of these programs work the same, using TAB and ENTER to automatically format scripts, your chapter on screenwriting software should give an overview of the options.
Write an introduction for your book. The introduction should include an introduction of yourself, including any experience you have as a screenwriter. This is important. Readers want to know the information they are getting in your book comes from someone who knows how to write screenplays and preferably has sold or optioned a screenplay or at least taught screenplay writing. The introduction to the book should also state the intent of the book. Don't just tell readers you're going to teach them how to write a screenplay. Explain how your book will do this. For instance, if you plan to illustrate with examples from scripts or supply exercises for scriptwriting, state this in your introduction.
Save your document as a PDF file for electronic distribution. Some word processors have built-in PDF-creation capabilities. If not, you can use a free virtual PDF printer to save as PDF. This will give you a file that can be distributed electronically. There are a number of other ebook formats you may consider placing your ebook in. Some formats are proprietary and can't be used without strict permission from the company who holds the rights, while other such as plain text, HTML, and Amazon's Kindle can be used to offer your book to a wider audience. PDF is a cross-platform standard that can allow you a greater audience for your book.