Contact a textbook publisher's sales representative and ask him/her if they can help you to do some contract work with their publishing company. Most sales representatives are willing to pass on your resume to editors and contract firms that work on publishing projects.
Get a small publishing contract. Test writing or companion website text writing are the two most common entry-level projects to work on.
Publish an article in a professional journal or a reference book. This sounds harder than it is; many large publishers, such as Thomson Gale or Pearson, hire subcontractors to help with reference books. Once you have a few writing credits, go on to Step 4.
Draw up your textbook proposal. Include a 3-level deep outline of the entire textbook. Write the introduction and the first three chapters. Describe who will collaborate, which sources you will use, and which graphics or images you will need. Compare your textbook to existing textbooks on the market, and explain why yours stands out.
Submit your proposal to at least three textbook publishers that publish textbooks in your field. Wait six to eight weeks, and follow up if you have not heard back.