The best way to teach nonfiction writing is to teach the most broadly encompassing elements first. The student has to have an opinion (called a thesis statement) before doing any other work, so writing instruction should begin with the thesis.
Next on the list to teach is structure. There are only two basic nonfiction structures you could teach, and of these two, the thesis-and-support organizational pattern, which is characteristic of persuasive essays, is the more useful.
At the tail end of items to be taught as part of the writing process are issues like sentence structure, grammar, and usage.
Both teacher and students should have a rubric for evaluating whether an essay is satisfactory. Rubrics foster learning by offering students direction and self-correction help.