For something like a design, I start by writing requirements, objectives, and then drawing artifacts: sequence diagrams, data model, evaluation of how the design meets the requirmeents etc.<p>Those fit into templates text like everyone else is suggesting, but I start by putting in nearby the drawings "figure x shows blah blah blah" once you've got that, you can introduce what that thing is saying, and after it, you say why it should be that way, usually referencing appendix material.<p>Most of technical writing I find is writing more and more detailed introductions. Each time there's a header, there's also a related intro statement.<p>Before you get far into the doc, writing out the glossary and defining the different actors is very helpful, so you can use that language consistently