Figure out what to write about
Understand the prompt or assignment.
The first thing you need to do is spend some quality time with the requirements of the assignment you're working on. What kind of essay are you writing? What exactly is the question or issue you need to answer? If you're not sure what the essay requires, you can always ask your professor to clarify. Before you begin, you need to have a solid sense of what the essay is supposed to do.
Know your purpose and audience.
This means that you need to know why you're writing, and to whom. Keeping straight the purpose of your essay will help guide you as you write, so you can stay on course, answer the prompt, and not stray into tangents. What are you trying to accomplish with your essay? What do you want your reader to get out of it?
This means you also need to know who your readers are. How much do they already know about your topic, and how much do you need to explain to them? Do they have any preexisting characteristics that'll affect how they read your essay or what they'll think of it? What tone and level of formality is appropriate for your audience?
The first thing that I do is read through my prompt and try to decide how I'm going to answer it. At this stage in my writing process, the only important bit is the first paragraph—the details will come later. So let's pick apart the prompt and see what I'll be doing.
In a 3-5 page essay, make and support a claim about how an image (picture, painting, advertisement, etc.) or a text (song, poem, speech, etc.) is being used. Your analysis should include discussion of how the image or text uses rhetorical canons (invention, arrangement, style, memory, and delivery) or rhetorical appeals (ethos, pathos, and logos). The purpose of a rhetorical analysis is to read, analyze, interpret, and argue your position with evidence.
Ok, it's a rhetorical analysis. Three to five pages. Pick an image or short text and analyze it—not the content of the piece, but how it communicates whatever message that it does. I should look at the rhetorical canons OR the appeals employed in the piece, but not both. That makes sense, for an essay of this length—I need to keep my focus narrow and not stray off topic.
The thing I'm going to need to keep most in mind is my purpose: I'm NOT looking at the content of whatever I choose to analyze. I'm strictly analyzing the WAY that the message is communicated. My audience doesn't want to know about the subject of the piece, they want to know about how the creator gets their message to the recipient.
All right, I think I've got the concept.