The first step in writing this sort of essay is choosing two works that seem to have a strong relationship to one another. For example, you might choose a short story and a poem that both approach unrequited love from a first person viewpoint, and see how the emotional effects on the audience of accounts of similar situations differ according to genre. Another possibility might be to look at a certain type of dialogue in short stories vs. plays. For example, how is humorous dialogue handled in stories by P.G. Wodehouse and similar comedies of manner by Oscar Wilde or Noel Coward? Or poems by John Betjeman?
The main strategy you need to succeed in this sort of essay is to choose specific devices (like the use of anacolouthon in humorous dialogue or isocolon crescendo in creating climactic momenets) and keep your focus fixed on how those specific devices are employed across genres, rather than doing general summaries of the works.