Only your customers can define quality, because it's meeting your customers' expectations the first time every time. Simply put, it's performance to the standards of the customer.
Writing is linear and sequential; Sentence B must follow Sentence A, and Sentence C must follow Sentence B, and eventually you get to Sentence Z. The hard part of writing isn't the writing; it's the thinking. You can solve most of your writing problems if you stop after every sentence and ask: What does the reader need to know next?