Form Romania is the decision that forms 
Romania. It can be taken by any nation with the Romanian culture (which means that it is actually not a cultural union). Those are 
Wallachia and 
Moldavia.
It follows the requirements of the usual formation decisions. The forming nation needs to have 45 prestige and own/sphere all the Romanian cores. It also requires the forming nation to be at peace.
Romania also have the alternative decision Union of Moldavia and Wallachia!, which is easier to take, but harder to keep the other Romanian nation away from taking first.