In TFH, since you don't generally start out at war with Nat China or the warlords, and you won't be called into war against Japan generally, you have a few options.
You won't be able to declare war on anybody (even if you consistently raise the threat of Japan of Nat China) until Japan declares war on somebody.
At that point you've got two options, you can try to back stab Nat chi, while they're in their hopeless war against Japan (in TFH Nat Chi, in a 36 start, will completely collapse rapidly 100% of the time). You will *not* be able to take territory from Japan, so you need to be careful to avoid going into areas where the Japanese occupation will be once Nat Chi surrenders. From there you should be able to capture all but 1-2 of Nat China's VPs (you can get the remaining 2 on the coast if you're daring and super lucky) and force them to surrender to you, giving you all the remaining parts of Nat Chi. From there you're free to invade whomever you want of the war lords - the only thing that will happen is that, due to your threat, Xibei and Yunnan will likely go Axis - and GiE once you conquer them. This means you'll have to deal with partisan activity in these areas if you go for them.
Note also that if you join the comintern while declaring wars (you'll want to get at least Xibei and Sinkiang after Nat Chi goes down) you will generate a tremendous amount of threat and completely derail the game. It's also entirely possible to get the UK to declare war on you before Sept 1st, 1939, which will mean a 3 way war. So, it's best to hold off on joining the comintern (the only way this hurts is that with the USSR leading your alliance, Nat Chi generally surrenders before all of its VPs are lost more often - but you'll also generate 100+ threat to nearly every country in the world simply by declaring war on Xibei and Sinkiang).
Alternatively, you can *not* attack Nat chi, and simply go for Xibei on its own once Japan attacks Nat Chi. That will open up Tibet and Sinkiang. The terrain out there is really rough, but you should be OK.
Things that can go awry:
If Nat Chi goes to war with the Guangxi clique, and it takes a bit, they will generate a lot of threat towards you, and you'll be getting a lot of goodies from it. If they don't fight, you will have a harder time getting enough threat on them to declare war when Japan does (or shortly thereafter). The date Marco Polo fires is also variable, so you may end up waiting up to a year longer on some games (which makes taking out Nat Chi harder, since you can't really build that much during that year). If it fires too soon, you won't be able to get enough threat to declare war on Nat Chi at all before it surrenders to Japan - making an invasion of Nat Chi. virtually impossible.
Lastly, you could just sit there and do nothing for years, and then once Germany is dealt with, attempt to deal with Japan - with Soviet help (if they survived).
In any case the biggest question you'll have, regardless of which conquest based route you take, is how you will help the USSR. Taking out Nat Chi will net you a bit of IC, and if Xibei goes Axis you'll still be at war with their GiE and get to keep your best laws after they fall. This will give you the ability (once you take out Sinkiang, or build convoys) to send lend-lease to the USSR. You can also send divisions (the easiest way to do this is just to set up a Theater command in Sinkiang, and give it objectives on Soviet VPs - since the AI doesn't like to accept expeditionary forces). You could of course give nothing, and if the USSR loses, you'll just have a bit harder game (but less American interference in your theater maybe!).
