Avanti Italia! is the decision that forms 
Italy.
It follows the requirements of the usual formation decisions. The forming nation needs to have either North Italian or South Italian as primary culture, have 45 prestige and own/sphere all the Italian cores. It also requires the forming nation to be at peace.
Although Maltese is technically counted as an Italian culture in the game files, Italy will not get Maltese as an accepted culture, and Malta is not required for the formation.
Sardinia-Piedmont, 
Two Sicilies and 
Papal States are the usual contenders.
Italy is often formed the alternative way with Girabaldis Redshirts.