The first step to writing a comparison and contrast essay is choosing the topic. You want to choose two things to compare/contrast that have both similarities and differences. Some students make the mistake of writing about only similarities or only differences because they forget to look at the larger picture. One has to be careful about choosing the topic because some things really don't have similarities.
Once you've chosen the topic, you'll next want to make a list of all the similarities you feel are important to point out and all the differences. You'll also want to take your analysis one step further and figure out why it is important for the reader to know about these similarities and differences. What further thing are you trying to convey with this analysis? Once you have a list of comparisons and contrasts and know exactly what you want to argue, you are now ready to compose the most important part of your essay, the thesis. A good formula to use for a comparison and contrast thesis is:
While both X and Y (strong verb, e.g. share) A, on the other hand, X is B while Y is C, showing us that (thesis assertion).
Once you have written your thesis and know the details of your argument, you can then decide how best to organize your analysis, and there are a couple of ways to organize a comparison and contrast essay.
The first way is Point-by-Point. Each paragraph will analyze one specific point and both compare and contrast the one point. If, for example, you were analyzing the quality of Mac vs. PC, one body paragraph could analyze software, comparing and contrasting both PC and Mac software.
The second method of organization is Block-by-Block. In this method, you will develop one paragraph that will be devoted to explaining one topic. The next paragraph will explain the same topic for the opposing side. Finally a third paragraph will be devoted to comparing and contrasting both sides of the topic. If we went back to our PC vs. Mac software example, the first paragraph would describe PC software, the second paragraph would describe Mac software, and the third would compare and contrast both types of software.