In the book Zen and the Art of Motorcycle Maintenance by Robert M Pirsig, a professor starts discussing what "quality" is with the students, becomes stuck, then spends the rest of the day at his desk trying to define quality, and finally gives up, deciding he has no idea what quality is.
We could just say "quality writing" is writing that you think is great and is suitable for they purpose you intend it for. But how often do we write something and dislike it or want to edit it endlessly? Let's talk about how to do your best work.
You can read books about how to write, but they are all suggestions you'll have to sort out for yourself. I definitely recommend you stay away from posts in writing forums that make up endless rules for writing. These are frequently bad advice. Consider the ridiculous "Mary Sue" trope that was popular about ten years ago. Don't even bother looking it up, just trust me that a lot of writing advice you see on the internet is way off base.
Taking a class or joining a writing group are good ideas because one of the best ways to improve your writing is to get good feedback and accept that feedback. Being edited can be difficult to take at first, so do it early and often. You can try to get a friend who is a good writer to look at your work, or join a class or group for good feedback. Sometimes you'll get feedback from a source that should be good and you really disagree with it. That's okay. Just let the advice sit and simmer for a while and come back to your work with fresh eyes weeks or months later. This fresh perspective almost always helps me accept whatever it was about my work that seemed off to the reviewer, or else helps me have time to come up with new ideas about how to achieve what I wanted, but in a much better way.
This putting away of your writing and letting it age before you do a final edit works wonders. I highly recommend it as a way of improving your writing. During that break time, your mind will have a lot of time to consider what you want your work to be like, and will probably even solve problems you had been struggling with.
The last resort is always to pay an editor. Bonus points for nice editors who try to teach you what their edits mean and how to improve. This is not cheap, but it's sometimes necessary if you want your work to be the best it can be.
Hope this helps!
--Pam