How should I approach writing a comparative essay?

It is essential to first check the weighting of the assessment criteria to cater your essay to the dominant assessment objectives. For example, in OCR English A-Level, 50% of the marks are for "understanding the significance and influence of the literary contexts", 25% for comparative analysis, 12.5% for "exploring interpretations of the texts" (i.e. critics), and 12.5% for "understanding of the texts and question".

Bearing this in mind, a comparative question on American Literature would be heavily targetting your ability to bring American literary movements into your argument. For example, Romanticism (1820-1860), which focused on individualism, idealism, imagination and nature, or The Gilded Age (1850-1914), which scrutinised the ills of society becoming increasingly multi-cultural during the age of industrialisation. To bring this into your argument, you will first need to identify ways in which certain themes are apparant in the two novels you are discussing. Let's use the example of The Grapes of Wrath and The Great Gatsby, and the essay title: "Poverty and the struggle to make a living are common themes in American literature." In the Grapes of Wrath, the biblical scope of the Okies salvific journey from penury to the promised land, and the arduousness of this process, could be contrasted with Gatsby's own dream of 'achieving' his American Dream, which Nick portrays as a divine aspiration. Not only is this demonstrating comparative analysis, you could then bring literary theory into your answer by identifying the relevance of The Gilded Age in both works. John Steinbeck examines the ills of a post-Depression era society  in which the poverty experienced by the Okies once they have migrated to California, is generated by those who have benefited from industrialisation. Similarly, Scott Fitzgerald examines how the Jazz Age signals opportunities for the elite to live a lavish lifestyle, but for the migrants condemned to living in the Valley of Ashes, there is much poverty. If you are then able to bring in interpretations that have been made of these texts, and show how these interpretations have changed throughout time, you will enhance your comparative essay even more. For example, in the 1960s the critical lens of ethnic minorities suggested that the migrant's struggle to make a living meant critics view Nick as being "surprised that black Americans could be successful or wealthy themselves" (Forrey), but this escalated in the post-modern era, when Marxist critics asserted that characters are so obssessed with material wealth they treat each other like objects.