#1a: The secret to clear writing.
The secret to clear communication, at least in writing, is to use simple sentences. The reader you aim to persuade is not intrinsically interested in your idea, so you must minimise the effort required for your idea to be understood.
Make every word count. Prefer simple words where possible. Find the word that exactly fits the context, and change those with meanings that could misguide your reader.
Like code, writing is read more than it is written. Optimise for your reader.
Also remember the clear writing comes from clear thinking. If you don't understand what do you want to say, what hope does your reader have?