Choose your topic. The Roane State Community College Writing Lab recommends picking items that share some link, such as two actors, two types of cleaners or two kinds of pets. Trying to compare things that have no clear connection -- like a book and a fish -- is more difficult but can be done, particularly if you want to make a satirical or humorous point.
Decide whether you want to focus on how two things are the same or different. Brainstorm all the characteristics you can think of for both, suggests the Roane State Community College Writing Lab, and then organize that information into lists pointing out similarities and differences. Choose your approach based on which list has more ideas and which seems more interesting or significant.
Compose your introduction. Kerry Walk with the Harvard University Writing Center suggests that the introduction for a comparative essay should contain three elements. The first element is a frame of reference, which may be a question, a problem, or group of related items that provides the context for your essay. The second element is the grounds for comparison, which is the reason why you chose these two particular things. The third element is a thesis that states your point. For example, if you write an essay comparing cats and dogs as pets, your frame of reference might be the problem of choosing a pet, your grounds of comparison are that these are the two most popular pets, and your thesis, or main idea, may be that they provide different benefits for owners.
Structure your essay using either the subject-by-subject or point-by-point method. According to the University of North Carolina at Chapel Hill Writing Center, with the subject approach, you analyze both of your examples, or "subjects," separately. For an essay comparing cats and dogs, you might devote one or two paragraphs describing the distinct benefits of cats as pets, and then devote one or two paragraphs to describe the benefits of dogs.
Use the point-by-point method if you do not like the subject-by-subject method, but don't use both. With the point method, mention the two subjects in each of the body paragraphs while focusing on one of the grounds of comparison. For example, in an essay about cats and dogs, you might say in one of the body paragraphs that dogs are better pets because they are attentive, they come when you call them and they love to walk with you. Cats, on the other hand, give you the cold shoulder, do not move a muscle when you call them and lie lazily on the couch when you want to go for a walk. Here, the two subjects are the cat and the dog, and the grounds of comparison are the pets' willingness to respond to you.
Write your essay. The traditional essay has five paragraphs: the introduction, three body paragraphs and a conclusion. After the introduction, us the three body paragraphs to support your main idea, or thesis. Begin your paragraphs with a topic sentence, which is a general statement that presents the main idea of the paragraph. Support that idea with specific examples, quotes and other evidence from authoritative sources. The conclusion paragraph sums up the essay. Restate your main idea and then leave the reader with something to ponder, such as a course of action or a striking quotation or image.
Reread your essay, looking for weak examples, unclear statements and sentence construction mistakes. The Purdue Online Writing Lab suggests reading your essay out loud and taking a short break between writing the rough draft and proofreading, to maximize your ability to find errors.