All technical documentation falls into one of four groups. I take the scheme from the Divio documentation system, which I first saw on Hacker News:
They form a square, drawn on the Divio page, in which each group resembles its two neighbors. Tutorials and how-to guides both describe practical steps. How-to guides and reference are both what we need at work, while coding. Reference and explanation both deal in theory. Tutorials and explanation are both most useful when studying rather than working. Seeing this breakdown adds structure to any documentation.
A few other pointers I keep. One Hacker News comment recommended the SEE method: Statement, Example, Explanation. An Introduction, a Summary, and a Rationale that names the alternatives make a document useful. Then there is Steven Pinker’s point about why there is so much bad writing, the curse of knowledge. You know so much about a subject that you forget your readers do not. Finally, be brief.