The only way to address writing is to give line-by-line feedback. We cannot assume that know what good writing looks like.
Actually, the first thing is to convince the student that he has something he (or she) has to write about. Then, when he (or she) writes that down, you go over it with him (or her). Then they discover how their mind works ... and start to know how to write. I say this from experience, because one of the few things I know I did well in my life was teach people how to write. Did they become great authors? Not so far as I know. But I am sure they became people who could engagingly write down what was on their minds. And probably pretty good judges of decent writing. I might add that I often think about my students and hope that they had great lives. You come to love people you teach.