Becoming a worker of words
Read deeply and broadly in the literature of the language (fiction and non) across time — taking in the rhythms of it, the cadence, the marshaling of words on the page, the words themselves.
Read poetry for insight into different strategies for writing concise, concrete, sensory. For scansion. And for the sheer beauty and pleasure of it.
Study a foreign language or two, to better understand what English is by seeing what it is not. Spend time with Latin for the clarity it brings to sentence construction in general and much of English vocabulary in particular. Delve into Anglo-Saxon for the heart and soul, the sound and sense, of English in its earliest days. There’s a music there you’ll want to make your own.
Take on sentence diagramming. Dabble in it at least, even if you don’t delve into it deeply. Though the deeper you go, the more surely will the intricate syntax of sentences unfold for you.
Study rhetoric for the discipline of it, and the principles. They’ll be useful in many arenas.
Study the strategies of both fiction and nonfiction, regardless of which you’ll be working in. The techniques of one often inform the other.
And write. If you’re going to help others write better — if you’re going to do more than simply correct mistakes — you’re going to have to know a good deal about writing yourself. You’ve got to have the knack of it, the feel of it. To know what works and what doesn’t . . . and why. If you’re to be an editor, always why.