what ways does jonathan swift use satire in "a modest proposal?'
First, Swift is satirizing the many proposals written by numerous, well-meaning reformers putting forth various plans to help Ireland and its people, many of whom are English and do not even understand the cause of Ireland's desperate situation--incredible poverty, over population, resources stripped for the benefit of England. And Swift, adopting the tone and logic of a typical "projector," provides yet another "modest" proposal to cure one of Ireland's most serious problems, over population.
Swift's elaborate, thorough, and logical discussion of the economics of his proposal, in which he thoughtfully includes ideas on the preparation of children for various meals, helps disguise the horror of his subject--that is, eating children as if they were meat.
The satire's real target, of course, is not the proposals he is imitating here but the English government, which has exploited the Irish people and land so completely that the Irish have only one marketable commodity left--children who must be raised and sold literally like cattle.