Oliver Miller responds (warning: harsh language) to advice given by writers in the Guardian on how to be a writer.
The key thing he mentions, to which all his other advice builds, is the need for good, constructive peer review.
So you need to surround yourself with fellow writers who are supportive but also honest. Some people will tell you that your writing is always good. These people are lying. And some people will tell you that your writing is always bad. These people are also lying. …But a few rare people will point out the stuff that they like, call you out on some of the dumb [stuff] that you’re writing, and gently but forcefully suggest ways to make your dumb [stuff] better [my italics]. Treasure these people. Learn to recognize them. These people are your only hope.
— Miller (2011): How to be a Writer