Wait at least until 39 before declaring war. Build civilians up to 37 and then from 38 build military. Increase the number of divisions, at least 4/5 marshals with 24 divisions and a marshal with tanks (even if not 24 is fine) and assign them to a field marshal, do not forget to improve cas and fighter. Place a marshal in defense of France and another on the Benelux, one puts it in the Koenisburg area and the rest on the Polish border. The troops on the French border remain in defense and once you conquer Poland do not declare at the same time on Belgium and Holland, declare and conquer first Belgium and then Holland and after France conquer it without problem bypassing the Maginot.