Form Poland-Lithuania is the decision that forms 
Poland-Lithuania.
It follows the requirements of the usual formation decisions. The forming nation needs to have Polish or Lithuanian culture as primary culture, have 45 prestige and own/sphere all the Polish-Lithuanian cores. It also requires the forming nation to be at peace.
Krakow is the only independent nation, that can take the decision, but 
Poland and 
Lithuania can be released and take the decisions as well.