Best watched from playlist : https://www.youtube.com/playlist?list=PL7sEehdlXVqTPGFMgUQdwLRzXAIao2a-V 1. Convert all your military factories to civilian 2. Build civilian factories until august/September of 1937 3. Build military factories until you hit desired number (I would suggest ratio of 100civ:150mil:50naval if you are interested in naval combat) 4. This strategy should fill up industry slots of any country by middle/end 1939 (be wary if you are going to be attacked earlier , this strategy is not viable - go for straight military factory production from beginning)
Hearts of iron 4 advanced guides - MinMax Industry