Ask HN: How can a developer become good at documentation

I have been coding all my life with most of the documentation limited to the comments I put in the code. But in my new role, I need to document the existing designs, architectures of the existing or proposed systems, prepare presentations for upper management etc. I am somehow getting this done but I don't see the end result as particularly appealing.

Are there any guidelines or a set of rules that I can follow. Also how can I get those nice graphics/flowcharts etc. into the documents, are there any good tools that anyone can recommend.