Research the number and types of books already published in the field to find a niche for your book. For your book, select a subject that no one has covered previously or no one has covered well. Generally, you will want to choose an area you are well-versed in, preferably one where you have received some critical acclaim.
Create a list of topics you wish to address within the book, a top-level outline of material that you will covered. Flesh out your topics with subheadings of the major elements of each broad topic, identifying your sub-sections for each chapter.
Compare the list of topics you wish to cover against college-level courses in your area, region or even nationally. Re-tailor your list if necessary to more closely pair your book to a specific course. This tailoring will drastically improve the chances of your book being used in a college curriculum.
Collect and analyze the latest research in all of the areas you plan to cover, making your book as up-to-date as possible. Additionally, consult professors at local universities who teach the subject you are writing about, asking them what they look for in a textbook (material and structure-wise). Writing specifically to address local professors' needs will help increase the chances of your book being used in the classroom.
Begin writing. Do not be afraid to revise chapters if new information is discovered while you are still in the process of writing. Remember that the longer you can go between releasing revisions of the book, the higher your profits for time expended will be.