Create the concept for the textbook you want to write. It needs to be structured and logical, but also accessible. Many textbooks published as part of the Great Source Language Arts series feature short lessons, designed to relate information in small portions.
Write your book. Include features such as opening section quizzes, student proofreading exercises, tips and lists of words or concepts that are commonly confused. For textbooks focused on vocabulary, Great Source books often have separate reference pages near the end of a section for spelling or grammar, as well as writing activities that let the students apply what they have learned.
Submit your textbook to Great Source. Include a one page statement that explains your approach and the educational philosophy behind it, another page that discusses your views of the work's prospects for sales versus possible competitors, a table of contents and one sample chapter and any other relevant materials. Do not send the entire manuscript yet. If they are interested, they will contact you.