i've never heard as "theory" referred to as a method. i think you're referring to rhetorical criticism? in which case, I suggest starting with Kenneth Burke. if you want to work with rhetorical theory (Classical Antiquity, continental philosophy, critical theory, etc), then you're method section would, I imagine, function primarily to argue for why your body of literature/various theoretical perspectives are compatible and relevant to the problem/issue/question you are exploring.