Once you know what you want to write and you've decided on one idea for the chapter or article, gather the material. That's called doing research. Learn everything you need to make your manuscript complete and include all essential information.
If it's a personal experience, search your memory and ask others who were involved. If it's historical or factual material (even if you write fiction) read widely. Find the one or two best sources—the original sources others quote.
Always learn more about a topic than you plan to use. Years ago I wrote a scene in a novel that included a woman's visit to a field of pyrethrum, a natural pesticide. By the time I finished my research, I could have easily written 5,000 words on the topic. In the novel, I wrote one paragraph and used 93 words. That's all I needed for the story.
When we research carefully, we provide accurate information. Keep records. Footnote your writing if needed. If you use on-line sources, verify the information before you quote.
Decide the anecdotes and illustrations you want to use. Think of those word pictures as windows. It's a way for readers to see inside the structure—to understand your statements.
Work hard as a writer so you can make it easy for a reader.