Form Yugoslavia is the decision that forms 
Yugoslavia.
It follows the requirements of the usual formation decisions. The forming nation needs to have a South Slavic culture as primary culture, have 45 prestige and own/sphere all the Yugoslav cores. It also requires the forming nation to be at peace.
Although Albania is technically counted as an Italian culture in the game files, Yugoslavia will not get Albanian as an accepted culture and 
Albania cannot form Yugoslavia.
Serbia, 
Bosnia-Herzegovina, 
Montenegro, 
Slovenia and 
Croatia are the countries needed. 
Bulgaria can also take the decision, but is not required for the formation.