Their rhetoric was considered skolion, twisted, because it appealed solely to the emotions of the hearer, and could bewitch or enchant the listener away from a righteous or moral path. So Aristotle's concern was to establish that rhetoric had a rational purpose by aligning it with the more scientific need for proof. This act distinguished the rhetoric of the day from the merely persuasive and poetic, aligning it instead with political speech. Prior to this, rhetoric had not been taken seriously as a form of political speech, as hard as that might be for us to understand today.
First, there is the appeal to ethos, establishing the proof of one's character. This is where one begins as an orator or writer, by establishing one's character and authority to be listened to and heard. For the Ancient Greeks, this was crucial, for to be taken seriously, a man had to be of good family, a good reputation, and have connections in the community. Nowadays, some of this is still crucial when writing, but the goals are accomplished differently in writing than in speaking. In speaking, an orator will most likely have a defined audience s/he speaks in front of, and will have developed her bona fides prior to speaking.
Her character will be established by the person who introduces her to the audience, who will tell you where she's from, what her educational background is, and any organizations she's affiliated with. All of these contribute to define the speaker's character. Yet, your character, your ethos, is more subtly communicated as well, through your tone of voice, your subject matter, and your presentation style. It is also communicated through respect for your audience, attention to their concerns, and thorough knowledge of one's subject.
When writing, the author establishes his character through his background, his vita, his list of works cited, and a biographical statement. It is important when you're a student writer to establish your credibility by picking a subject you have personal experience with. It is impossible to take a writer seriously when s/he chooses a subject she has no experience with. Your experience is your avenue to credibility. If you choose to speak or write on a subject you know nothing about, you will feel uncomfortable and you lose credibility.