After deciding what to compare, write down what you know about subject A and then subject B. Next, divide a page or use your software to create a table with three columns (up and down) and at least half a dozen rows (across). Use the first row to label the columns:
| Categories | Subject A | Subject B |
Now read over your notes on subject A. When you spot related details, identify a logical category for them. Enter that category name in the left column of the second row. Then add related details for subject A in the middle column. Repeat this process, labeling more rows as categories and filling in corresponding details for subject A. (Draw more lines, or use the menu to add new rows as needed.)
Next, review your notes on subject B. If some details fall into categories already listed in your table, add those details in the subject B column for each category. If new categories emerge, add them in new rows along with the subject B details. After you finish with your notes, round out the table—adding details to fill in empty cells, combining or adding categories. Select the most promising categories from your table as common features for logical comparison and contrast.