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You can create it in the Age of Imperialism, for example. I think it happens automatically after a certain date when one of the Italian countries controls Rome and other major cities.
In AGCEEP, you need to be a Monarchical State (ie not Genoa or Sienna or Venice), you have to own most of Northern Italy province (except Venice I think...), a low badboy (so diplo-annex) and be friend and ally with the Pope or the Emperor. All of this before 1520.
Milan is the easier way to achieve this.