Games

I've always just dived in head-first and learned the hard way. Usually the easiest way to learn is chosing one of the non-aligned minor power countries and playing around while letting the major powers do the brunt of the work. Hell even starting off as America is usually good practice because it's darn near uninvadable and you have plenty of years to play around with building up, setting up divisions, sending volunteers or arms, etc. And nominally just being able to take it easy while learning.

And of course being willing to take losing games to their conclusion so you can get to experience the crunch of desperately trying to stave off defeat.

Of course things probably changed quite a bit since I played, but the key fundamentals I remember is try to time building your industry in peacetime while gearing up for war, i.e estimating you have roughly 3-4 years of peace meaning building civ factories for a year or two then building military factories in the remaining time while regearing you current mil factories to building the basic infantry gear to get your divisions fully supplied while training new divisions with the surplus, and only really spending time building stuff like tanks or planes exclusively if you have the surplus factories to do so.