Choose a subject that you have expertise and there's a need. Consider what you expect the class to learn and how; decide the length of the class; and consider your audience and purpose for writing the class.
Choose a format and create an outline. A simple order could be table of contents, introduction, lessons, exercises, glossary, index and credits. Choose your reference books or create reference packets. Block time for research.
Create a list of objectives for the content of the class and the students. State why the student benefits from the class, how your class differs from others and provide supportive evidence.
Write the class, adding illustrations as you progress from section to section. Edit each section as you move on to the next. Use American Psychological Association style documentation within your document. Try all the exercises yourself, and test the class on friends and family to ensure its effectiveness.