The Embassy of Italy will give you information with the working holiday visa issued to you on how to apply for your working holiday permit to stay or permesso di soggiorno once you have arrived in Italy.
To avoid difficulty obtaining your permit, as a visa holder you should apply at the Questura (Police) in a major town or city nearest to where you will be staying and not at smaller police stations which may not know of the existence of the working holiday scheme.
In Rome, e.g., go to the Questura Centrale, Dirigente per l’Immigrazione, Via di San Vitale.
The same scheme offers young Italian people the opportunity of a 12 month stay in New Zealand on similar terms. For more information and to apply see the Immigration New Zealand website(external link).