Begin the lesson with an introduction to the author and the context of the piece. You can choose how much detail you give your introduction to Swift's life, but you will definitely want to point out the outrage he felt at the way that the British government treated the Irish, particularly the poor. During Swift's time, the Irish sought to join England, but were not permitted to, and the way England dealt in commerce with Ireland led to terrible living conditions for many, simply because economic opportunity was nonexistent.
"A Modest Proposal" was just one of many essays that Swift wrote during the 1720's, as he became passionately involved in Irish politics. Swift employed sarcasm which, at that time, was not nearly as intertwined with social discourse as it is in our own time. As a result, most of his rhetorical argument went misunderstood. This essay would actually be the last that Swift would write about poverty in Ireland.
To help your class establish context, as a warmup at the beginning of class, ask your class to jot down the two or three most important political issues in the country. Then, ask them to jot down a sentence or two detailing a way that they would solve these problems.
Then, move into a whole-class discussion of what people thought of. In a junior- or senior-level English class, you should get some interesting responses.