I just want to know what is the best way to get stable income as horde
Hordes are intended to be unstable. They have highly autonomous land, so they don't receive much income from taxation and production (like other governments do).
Therefore, you need other sources of money:
(1) Gold Mines.
The easiest mines are in Buryatia and in Yarkand—convert the latter and develop both (for Renaissance and Colonisation).
Also Japan may get one-two gold provinces as an event.
Also somewhere in the Nothern West India there is a gold mine (Mewar, as I recall).
The major minus is the increasing inflation which makes your troops cost more.
(2) Trade and Trade Companies.
This is rather hard for a new player, because Trade system is rather complicated in EU4.
General Advices: conquer into Samarkand -> Persia/India and collect money at the end of your chain;
force your vassals to Transfer Trade to you;
move your capital to Europe and add Indian provinces to Trade Companies.
(3) Looting:
Isolate several Tributaries of Ming from each other.
Leave each of them with 4-6 provinces in
different states (It may be my bias, but AI tends to develop more if it has two states),
Declare war and loot their provinces while not paying for your troops.
Take only money and War Reparations in the peace deal.
Come for more money in 10 years.
4) Non-horde vassals.
Feed them with newly conquered lands.
Remember to delete forts before giving the provinces, so your vassal does not get into a debt spiral.
5) Debasing.
Look for EU4 Debasing guide at Youtube. It explains why and when to debase.
And a guide recommending you never raze as Horde is incredible in its own unique way.
I tried similar run when the 1.20 patch came out, although I did raze some provinces: those which I did not want to statify. Still, it did not help much with money issues as a horde.
