To do a rhetorical analysis of nature, you should list out in a word processing file a series of major rhetorical strategies, and then, as you read through the essay, note each time you find a particular type of strategy with a short description and a page number. Once you have finished reading the book, then you will be able to flesh out each section into a paragraph, and produce a solid essay.
Begin by dividing rhetorical strategy into the canons of argument and style.
For argument, look for the three forms of proof (from reason, character of the speaker, and emotions of the audience), inductive and deductive reasoning, and common topics. For common topics, Emerson frequently argues from cause and effect (often in the form of using the origin of X to argue for the quality or use of X) and from analogy (cf. Joseph Butler's Analogy). One form of argument from analogy he favours is what Aristotle calls the topos of lesser and greater.
For style, look especially at how Emerson uses parallelism to emphasize analogy. He will often use a structure known as a tricolon crescendo (A is good, B is better, and C is best) to proceed from the material aspect of nature to nature understood spiritually to pure spirit.