Dealing with Dai Viet
In the first month of the game, 94% of the time Dai Viet will declare war on you via event. So move your armies south in January to prepare, but keep maintenance low. The army you have in Vietnam should move south, separating out the cavalry to run ahead. Don't engage in battle with it. You want them sieging from the south on up, where the AI will leave them alone to siege.
Early on, don't fight with Dai Viet. They will siege Guangxi; let them. Retake it once they leave. Dai Viet haae a leader, Le Loi, with the stats 5/4/5 -- one of the best leaders in the game. Even using Yongle (4/3/3), Le Loi will beat you practically anywhere. The best hope to kill him is to catch him in plains with your full cavalry army, especially when he doesn't have that many men with him. Sichuan Pendi is great for this, since it has no fort. Dai Viet will take it, then move back into Vietnam; then you retake it and run, and eventually they'll send an army back out to take it again. You pounce then. Another good place to confront Le Loi (and lesser armies) is defending in Guizhou: it has high support limits, and you'll be defending across a river and in mountains.
Your goals for the war should be, first, to use up all the enemy's starting cash building armies. Kill enemy armies when you can cheaply, so that he must keep building. Second, kill Le Loi. Do not wade into Vietnam itself (other than your southern army), until Le Loi is dead. Once Le Loi is dead, you want to move on to a "normal" EU2 war, where you eliminate the enemy army, cover every city, and siege them all down to get 100% warscore.
When you get 100%, do not immediately make peace. Instead, prepare for synchronized looting, and stay at war with Dai Viet at least until you are done with your initial inflation. There's no rush to peace, since your losses should be perhaps 20000 infantry and 5000 cavalry with a proper war. When you finally do make peace, you can decide whether you want to vassalize Dai Viet, perhaps for later diploannexation, or whether to take their three provinces. It is a defensive war, so taking them is cheap in terms of badboy.
Starting Your Economy
On Jan 1, don't forget to drop maintenance spending, set your religious tolerance, and move your domestic policy slider. I suggest dropping innovative by 1, so that you get 0.4 missionaries per year. You will want missionaries for conversion attempts.
Your first goal, as with any country, is to get Infra 1 and promote all your tax collectors. Infra 1 takes just over a year. Once you have it, spend all your cash on tax collectors, and also take one loan immediately, and one additional loan each subsequent December, so long as you still have provinces with at least 4 base tax value without TCs. (If you build tax collectors in December, then you get the census tax the next year, and the loan repayment dialog can be left up until the end of the month, allowing you to pay the loan with census taxes on January 1.)
Once you get all tax collectors in place, get your stability up to 3 and then inflate up to 2% inflation.
You have good Admin monarchs only for the first bit of the game, so garrison and send a missionary to Xinjiang and Yunnan. (Don't do this until after you've eliminated Dai Viet's armies.) Yongle (ADM=8) dies in August of 1424; you get 2 missionaries before then so both provinces can be attempted. Hongxi (AMD=7) dies in May of 1425; Xuande (ADM=6) dies in January 1435. So you should be able to send 3 missions to each province, with around 50% chances.
You start with Zheng Ho, who is among the best explorers in the game. He lasts until 1436. You can certainly use him in a more-or-less historical way, to discover some of the South Asia coastal areas. But you can mapswap for all of this except for empty provinces, and attack people for maps to get that.
It is much more profitable to use Zheng Ho to discover America. This can be done in either direction. To the West, you've got to spend diplomatic money to exchange maps and buy military access to open the route through the Indian Ocean to Zanzibar, explore around Africa, buy MA from Portugal and explore across the Atlantic.
To go East, you get MA with Nippon, then explore Arctic waters to California. After discovering, clearing, and colonizing Sacramento or Monterey, explore south to Central America. Then get military access with the Zapotec or Maya, and push south to the Chimu and Incas. An expert player can find and conquer both the Aztecs and the Chimu/Incas before the Europeans do more than place a few trading posts in Brazil.
Early Military Expansion
Your initial expansion goals in almost any game should be to grab all the provinces of your state religion that you can. If you've got a large state religion, like Catholicism or Sunni, then you cannot grab all of it without exceeding the BB limits. So you'll want to be selective, only taking rich provinces and same-culture provinces of your religion, which are the most productive. But for China, there's no need to be selective. There's only 18 provinces of your religion that you don't start with. Go get them.
My suggestion is to form an alliance with Nippon ASAP. This will probably require a royal marriage first.
Now attack Manchu. (You have permanent CB on them.) Try to get Nippon to complete the siege in Nakjodka; you can also let them control the other five provinces you don't have cores on, although it doesn't matter either way. For peace, you should take Baicheng, Jehol, and Liaotung; you also want Nakjodka to Nippon, and you can give other provinces to them if you want, but I don't recommend it, as it is quite easy to diploannex a five province Manchu.
Why Nakjodka to Nippon? Two reasons: first, it means you will border them; a border is required for diploannexation. Second, it gives Korea an adjacent province to attack, when you attack them. This makes it much easier to conquer Korea, as versus attacking a huge stack sitting in the mountains.
As soon as you get peace with Manchu and your WE drops to zero, your second target is Korea. DoW, and use Nippon as an annexation ally to annex one-province Korea after you've made a separate peace taking Kyongju and cash. Japan will now own Yalu; this gives you a permanent CB on Japan.
Now build up a fleet of galleys, perhaps 20 or so. You'll need these for the attack on Nippon. Since they've got Yalu, you've got permanent CB: throw them out of your alliance, DoW, and take them over. For peace, take Yalu and vassalization. This leaves no CBs either way; you can diploannex Japan in 30 years.
When your truce with Manchu expires, complete the job: your goal this time is vassalization of Manchu for diploannexation in 10 years.
The Strategic Decision after Zheng Ho's Journey
Sometime in the five years from 1435-1440, you'll face a scripted event that has huge consequences for your game. The event is The Strategy Decision after Zheng Ho's Journey; and it can radically alter your domestic policies.
For a game of world conquest, arguably the first choice ("Inward Perfection") is the best; it has these domestic policy consequences:
Domestic Policy Initial Inward Perfection Result Aristocracy 9 +10 10 Centralization 7 -5 2 Innovative 5 - 5 Mercantilism 5 +10 10 Offensive 4 -10 0 Land 6 +10 10 Quality 5 -10 0 Serfdom 9 +10 10
Also it gives some investments into land and infra tech, and stability +6. The reason this choice is good for world conquest is that for world conquest in 1.09 you need minimum centralization and innovation to keep maximum war exhaustion down. You also want low stability costs, cheap armies, and low offensive (to get the +1 siege modifier). Most of these desirable properties are obtained with this single event.
The b choice ("Balanced Position") may also be good for world conquest:
Domestic Policy Initial Balanced Position Result Aristocracy 9 +1 10 Centralization 7 +1 8 Innovative 5 +1 6 Mercantilism 5 -1 4 Offensive 4 +1 5 Land 6 -1 5 Quality 5 +1 6 Serfdom 9 +1 10
For any non-world conquest game, you'll want to choose choice c, "Outward Expansion". This choice has some nasty effects (-6 stab and 5 years of +8 RR). But it moves you into the muslim techgroup, which is much superior to the china techgroup. Its domestic policy effects are also good for a colonizing country, giving you good investments to make while you develop tech.
Domestic Policy Initial Outward Expanson Result Aristocracy 9 -3 6 Centralization 7 +3 10 Innovative 5 +5 10 Mercantilism 5 -5 0 Offensive 4 +3 7 Land 6 -5 1 Quality 5 +5 10 Serfdom 9 -5 4
Dealing with the 1644 Events
The best "line" through these events is to make sure that the Manchus exist by April, 1644; this avoids the nasty Emperor Chongzhen Commits Suicide event. You can free Manchu as a vassal just before April.
Then in the Fall of the Ming Dynasty event, choose option b ("proclaim the Qing Dynasty"). This gives Manchu an event, where they will almost always (94%) choose to join China, that is, you inherit them. (There is also the small chance (about 6%), that you get a supernasty event causing you to cede most of China to Manchu and become their vassal. Woops! Have fun...) Next, an event causes you to become Manchu; you will lose Cantonese culture and gain Manchu culture in this event. In January 1683, you (as Manchu) get an event where you can choose to become China again and regain Cantonese culture.