<p>I've dissected this more than I'd like (and it's sad that I'm letting all my built-up knowledge go to waste, as I no longer have time to write). If you're talking about creative writing...</p>
<p>To me, a good writer is one who knows how to structure his or her writing, from the phrase level to the novel level; who uses words well (this includes using them in an appropriate way and at the right time); who makes every sentence as concise, precise, and clear as possible; who employs the main types of writing appropriately (dialogue, description, action, senses, dramatic summary); who can paint characters in words; and above all, who can achieve the following in his or her writing voice: confidence, humbleness, eloquence, and ease.</p>
<p>You become a good writer by reading. When you read, you accomplish a few things: you increase your vocabulary (the elements of language, your tools); you hone your "language sense," as in proper grammar and syntax (the principles of language); and you see the levels of structure on the sentence, paragraph, scene, and story level. It's possible to achieve the first two through active pursuit, but the latter must really be observed in well-written books: seeing how a scene progresses, how dialogue is integrated into the story, how and when something is described and to what detail.</p>
<p>And of course, you become a good writer through writing more. You'll obviously read more than you write, but a large part of a developing ability is trying yours out and seeing what happens. You'll find that your style is built off the writers you read, but it'll be different.</p>
<p>It's a complicated process, more complicated than I would like, but the finished product is well worth it, whether it be an essay, a research paper, a screenplay, or a novel.</p>