Study the summary requirements carefully. This could be the question or instructions which set the assignment, or a list you make yourself of what the summary must include. Make sure you understand exactly what you need to do, and if you are responding to a question, write this somewhere where you can see it throughout the planning and writing process. Refer back to the question or criteria regularly to ensure you stay on topic.
Draw a Venn diagram (two large circles which overlap in the middle) and label each circle as one of the themes you are comparing. The overlapping section is for the similarities between the two themes. In the rest of the circle, write differences about the two themes. For example, to compare cats and dogs, you would title the left circle "Cats," and the right circle "Dogs." Characteristics of cats would be in the left circle, and dogs in the right, such as "cats are good hunters" and "dogs can be reliably trained." In the overlapping section would be similarities between the cats and dogs, such as "both are kept as pets," or "both walk on four legs."
Use comparative tables. If you are comparing places or businesses, have columns to compare location and price, etc. For theories and ideas, compare who created them, when, what are the central ideas, etc.
Make a list of all of your comparisons so you can see them all clearly. Read through the list, and cross off any that aren't relevant to your summary title.
Summarize the information you have. Looking down the list, what are the differences that really stand out, or that are unknown? For example, when comparing restaurants, choosing one that uses fresh food and another that does not would make for an interesting contrast.
Use comparative language. This will help to guide your reader through the summary. Phrases such as "In contrast to" and "like" will help the reader to understand each point, and make the summary flow better.
Read through your summary to ensure that it is concise, informs the readers of key differences between the two themes, and is easy to read and understand. Ask other people to read it, too, to ensure it makes sense.