So this one is pretty simple. Just a matter of surviving, which as Poland can be a little tricky.
What worked for me:
- at start of game, put your political power into going Communist - i was able to achieve entry into the Comintern by 1938ish - when you end up at war with Germany you'll have the Red Army come streaming in to hold the line. (plus France and UK often still guarantee your independence.)
- I disbanded all the starting Cav, upgraded all the infantry to 7inf, 2art + support arty, engineers, hospital, anti-tank, recon. by the war had the starting inf upgraded and trained to regular and about 15/20 more infantry divisions not far from produced.
- focuses, did all the economic ones first, down to the extra research, then the military ones down to the focuses that give you land forts. then the political ones.
- keep in mind if in '39 you think you are not ready for war you can give up Danzig for a short reprieve (though obviously you lose the factories there), if you give up danzig this way and the germans dont DoW you reclaiming Danzig is a very quick justification. if you want to get into war anyway.
- When you end up at war with Germany focus on holding ground on the West (give up a little if you have to), and prioritise closing down the pocket to the North - keep in mind, to cut off supply you will need both Konigsburg and Memel)
- I started researching the rockets in 1940ish. and was done by 1945. in the air focus tree the final focus gives you research speed boost for rocketry, (maybe x2?) which can have the research done in as little as 150 days.
Good Luck!