Write an outline for the entire textbook series. Think about how you learned the subject yourself or how you currently teach the subject. Start with the basic concepts and build from there.
Write an outline for each individual book. Decide what content you will write yourself and what content you will need to obtain from others. Just because you cannot write all the content yourself does not mean you cannot publish a textbook.
Contact a self-publishing company that has experience publishing textbooks. A company like Dog Ear Publishing (link below in the Resources section) has editorial experts on hand who can assist you with locating missing content or securing the rights to additional content. The self-publishing company may even be able to find content that is in the public domain, which means you will not have to pay for including it in your book.
Work with the self-publishing company to design the layout and cover for your textbook series. Communicate your vision clearly, but also listen to their advice. You're paying for their services and their experience.
Develop and execute a marketing and distribution plan. If the self-publishing company offers this service, consider leveraging their experience and contacts, although there may be an additional fee. You'll want to contact professors, vocational colleges and schools, school libraries, and other institutions that may be interested in the subject of your textbook series.