I'm a professional writer and editor. I regularly hire technology experts like you to explain things to other tech experts.
There are several paths you can follow. I would start out by writing for yourself, first. That could be a blog or even a daily journal. (The Artist's Way by Julia Cameron is recommended.) At this point don't worry about what to write; just get yourself into the habit of articulating your thoughts. Nobody needs to see this, just as they don't need to see your first Hello World efforts with a new programming language. It's okay to fail. In fact, it's necessary.
Another reason to write first for yourself is to discover what you like to write and what you're good at. (Ideally these overlap!) By writing for myself, first (originally in computer user group newsletters) I taught myself how to write a product review, and if I say so myself I became damned good at it. (Well, I don't have to say so myself; these days I'm a top reviewer on Amazon, and it's just a hobby.) You might discover that you like doing "follow the bouncing ball" how-to articles, or that you find them painfully difficult. You may discover a fondness for writing round-up articles (5 bug tracking tools) or reviews of programming books. Or a really effective rant. Whatever... the point is that you should try to write in different categories.
It won't take long before you begin to recognize your writing weaknesses, even if you don't know how to fix them yet. Again, like software development: You could realize that you tend to write software with memory leaks, even if you don't know the skills to fix it yet. But seeing what needs to be improved -- at least in your own head -- is important. Because if you think you're perfect, you won't learn from others.
Only then should you start to get feedback.
Whew, this is already pretty long. Let me know if it's on the right track, and I'll keep going.