Everyone writes, then does a better or worse job of editing. Like code, prose tends to crystalize into smaller and higher quality pieces over time.
--- Process ---
Write all of your thoughts in a stream of consciousness flow, don't worry about how it looks, just get your thoughts out of your head, and into the storage medium of your choice. This frees up stack space in your brain.
Next - Iterate. Strategies you can use to help include:
Read it out loud to yourself. You'll immediately notice typos, grammatical and flow issues.
Walk away from it to gain some distance in time and space. When you come back you'll notice gaps or repetitions.
Repeat until you're happy.
Seriously, set out the main points you want to state as a list. Then flesh those out. Then start editing (as one of the other commenters has already said). Think about what you have, maybe change the layout and edit some more.
You may need 'one version to throw away' before you come out with the final product. Very rarely will you be 'finished' when writing a document. There always seems to be just one more edit.
Don't let the fact that the document is 'unfinished' delay you if need be. Sometimes you just have to get it 'out the door', unfinished or not.
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.
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.
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
It is like periodic table for chemical elements, singularly the one best thing about documentation that I ever read. It helped me immensely to structure the way I am thinking about documents. It starts with clear delineation of four quadrants: tutorials, how-tos, explanation and reference, and everything just clicks from there. I found that my problems usually due to subconsciously mixing them up - and setting the boundaries is very useful.
I also found Jordan Peterson's essay writing guide [1] useful, not just for academic writing.
[1] https://jordanbpeterson.com/wp-content/uploads/2018/02/Essay...
After a while, you find patterns that make it more effective and easier.
I always start with a nested list of what I want to communicate. I refine the order and completeness of this list for a bit, usually over breakfast.
Once you have that, you just flesh out each point one by one.