Ok, so here are some import points:
-Train your troops until they are level 3.
-Make 7/2's (they're not meta anymore blablablablablabla, i know but you're poor as fuck, so this will have to make do), and slowly update your previous trash infantry and cavalry divisions to them.
-Hire military advisors, specifically the army logistics expert. The difference when having him on is night and day, and makes pushing into Mengkukuo much, much easier. Other recommendable advisors include:<Infantry expert.<Army Regrouping expert.<Entrenchment expert, if you plan on grinding them down for a bit (which is usually what i do until i can update my divisions to 7/2), though it isn't obligatory to hire him.
Chief of army is either the Army Morale expert or the Army Defense expert, i'll leave that up to you.
It is also recommendable that, if possible, you decrypt Japan's cipher, as well as launch the 100 regiments offensive. It's really helpful when pushing and seeing their shit fall apart.
When joining the war, you can take one of two approaches:
1. You join it after setting up your defenses in a fallback line on your border with Mengkukuo and gaining entrenchment bonus, this ensures the nationalists don't fucking die, as some enemy divisions will stay there and won't maraud the KMT as much as they normally do. Time will be mostly spent defending (doing some pushes here and there whenever the AI does its thing and forgets to man up/barely mans up some tile), but also focusing on upgrading your division templates and building stockpiles. When you feel the time is right (your divisions are strong enough/theirs are weak), you strike and seek to overrun Mengjiang. At this point, they will start to fall apart, and killing Manchukuo shouldn't be a problem (just watch for the AI trying to encircle you, as at this moment you'll probably be snaking quite a lot).
2. Do NOT join the war immediately, you want the nationalists to do a lot of the initial heavy lifting (but don't let them die either). Only do so when you're fully ready (i.e. done everything i said so far), at which point the Japanese should be starting to stretch their lines, and you can overrun Mengjiang swiftly depending on the circumstances (the 7/2's will trash what the fascists put there), and even cut their forces in half if you manage to get to Qingdao, though no guarantees.
If you push the Japanese army out of the continent, you should get an event where Japan offers you a white peace, and you prepare to turn on Chiang Kai-Shek. However, i never got it since the DLC is bugged as fuck, in which case, simply build subs (preferably by getting some other country's license production/lend-lease), naval bombers, raze the IJN to the bottom of the ocean, and invade Japan. Its army will be very pathetic given the multiple naval invasions you repelled during your preparation (tip: the AI loves trying to invade the Korean peninsula, so you can either repel it each time, or you let them get a bit of a foothold, and destroy whatever troops they mindlessly shuffle in it).
Also, try to encircle their troops whenever an opportunity appears. It takes a medium amount of time to see their army bleeding, but it is well worth the effort.
Make plans to create planing bonuses, these can be handy buffs in battles.
If the war with Japan takes too long (i'm talking 1946 level long), it is possible that the KMT will start doing the One China Policy focus, which will makes them immediately attack you, even if in the middle of the war. So keep an eye out for that.
The guide i used to properly learn how to play is the embed video.