To get started, I would suggest that you focus on one of the three characters of the story: the narrator, his wife, or Robert. Begin to do some brainstorming on the characteristics of this character and how he or she changes or does not change over the course of the story. Then, begin thinking about how Raymond Carver communicates those characteristics and changes (if present).
If needed, you could always add another character to the mix if you feel you do not have enough information for a five to seven page paper. My suggestion would be to start with the narrator as I believe he will provide you with the most opportunity for comment.
While you are composing your essay, be sure write a developed paragraph of introduction and conclusion. In your introduction, include the title of the story, the author, the type of story, a brief identification of the main characters, and a brief explanation of the general situation. This information will ground your reader in the story. Then write your way into your thesis, the last sentence of the introduction.
In writing your conclusion, see if you can surpass the basic summary conclusion and reach for the "So what?" conclusion. This requires critical thinking and interpretation. You have explained and supported a thesis statement, made your case, and now ask yourself, "So what?" What is the significance of what you have established? What conclusions can you draw? What observations can you make based on the story, character, etc.? Impose some meaning upon what you have revealed and established in your paper.
Finally, in developing your main body paragraphs, use facts, examples, and incidents along with the passages that you choose to quote or paraphrase. Including a variety of supporting evidence, especially the summary of incidents, will help you meet the length requirement. Good luck!
You've picked a good topic for this particular story, because the narrator in the short story "Cathedral" by Raymond Carver does experience quite a change of perspective for his character. To write a good thesis sentence that will help to guide you on your entire paper itself, I might consider filling in the blanks on a sentence similar to the following: "The narrator in Raymond Carver's 'Cathedral' develops from being _________ to __________ ." You can try filling in the blanks with simple, one-word ideas, like "blind" to "seeing," (which might present an interesting twist, and you could discuss how the narrator was blind to anyone else but himself and his own perspective, and then, through the help of a blind man, was able to open his eyes and "see" life through a different perspective, which changed him). Or, you can try filling in the blanks with longer phrases like "a bitter, cynical man that found no joy in life" to "a humbled, open-minded man who could see joy in small experiences."
To help track his development throughout the story, list his personality traits that he displays at the beginning of the story, then some that surface in the middle (his patience with Robert, his willingness to sit up with him late into the night, talking, etc.), and then what traits he seems to gain right at the end. Then, as you write your essay, try to talk about several traits in each category, AND tie them with quotes directly from the story itself to support your assertions. For example, if you picked that he was a rude man at the beginning, you can use the quote where he found the death of Robert's wife "pathetic" because so little money was left for him. That's a pretty horribly rude thing to say about a death. If you do that for all of the traits that you pick, you will be able to get a decent length out of the paper.
I hope that these thoughts can help you to get started on writing. To summarize the steps: 1. Fill in the blanks for the thesis statement. 2. Write down 2 or 3 personality traits for the narrator at the beginning, middle and end of the story, and 3. Find quotes from the story to support all of those traits. Then, if you have all of that information, you are good to just piece it together in an essay. Good luck!