Ask HN: How to be good at technical writing

Here is my story, I was never that good at technical writing. I figured it out couple of years earlier, when I was writing my MS thesis.

During my MS thesis I had two supervisors, one is from the company I was working with and other supervisor was my university Prof. My company supervisor was happy with my work (his responsibility was to guide me in hands on work) and my professor was responsible for checking my thesis paper. And my professor was not that happy about writing. And eventually I got an average grade in my thesis. My professor used to say "the thesis needs to be more clear, in lots of places explanations of "why" is missing. I thought it's fine, its enough to write good code build cool system.

After working couple of years, starting this year I got promotion and now I am leading a small dev team. Which leads me to write lots of design docs, before starting any new project/service.

And I have a feeling that my design docs are not as clear it should be. How you guys write software design documents? I know writing a lot helps, I even wrote some blog posts in company engineering blog but those are specific to a problem or tutorial kind of writing. So, how one can get better in technical writing? Any book/blog recommendation will be greatly appreciated.

Thanks in advance.