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I've written & published fiction for yeas and have edited others' writing for years. In order to be an editor you'll need excellent grammar skills as well as a knowledge of stylistic skills--i.e., learning how to vary the rhythm and music of sentences, i.e., cutting irrelevant or repetitive sentences, acquiring a knowledge for cutting 'telling' sentences. Readers want to enter a world where all the senses are in use. There's so much more, but this is a good starting place, I think.