Use What You Know
When people are stuck on what to write about, I always tell them to write about what they already know. This could be what they went to college for or it could be job related. You can even create textbooks based on your hobbies. Make a list of all your skills and interests.
Know Your Market
The next step in writing a textbook to self publish on Kindle is to check out the market. Do some research on your topic on Amazon.com. See what others are publishing, check out their sales rank, and read the reviews.
Choose Age Group
What reading level and what age are you writing your textbook for? Will this be for self learners, classrooms, or homeschooled children?
Study Other Books
After you have decided on a topic and who you will be writing the textbook for, your next step is to research other books on your topic. To understand how to write for your age group or grade level, go to the library and read books that are in those age groups.
Photographs and Illustrations
All textbooks should have illustrations or photographs. Original is best. If your textbook includes projects, include photographs and illustrations demonstrating the project. Graphs and charts are also a great addition to textbooks.
Do your research, create your outline, plan your projects, and get writing. It is one thing to dream about writing and self publishing a textbook, but it is quite another thing to commit to the work. Get motivated, create a writing schedule, and get it done.