Practice. Pick up texts on your own. Can you trim a 1,000-word article by 10 percent? 20? 25? Can you do it with a scalpel, excising unnecessary words and phrases? Can you do it with a bone saw, removing whole paragraphs that do not contribute? Can you sharpen the focus of the opening paragraph? Can you get to the damn point up front instead of permitting the writer half a dozen paragraphs of throat-clearing? Can you untangle mixed metaphors and tone down the writer's stylistic excesses? Can you vet statements of fact for accuracy? Can you restate technical language in common language and eliminate pretentious jargon?
At the end of every semester I quote Chaucer to my students at Loyola: "The lyf so short, the craft so long to lerne." Becoming an editor is a life's work. After more than three decades as a professional editor, I find out new things about language and usage nearly every day. Every time I open a text for editing, I wonder whether I will identify what needs to be done and whether I will overlook something important. I will never know enough, and I will never catch everything.
I will not give up trying.