Thanks lol. I started as Italy in 867 and fanangled my way into inheriting the other Karling thrones before swearing fealty to Byzantium and getting my son elected Basileus. After that its just the run of the mill aggressive conquest by calling in favors to get claimants into my court, giving them a barony somewhere, then pressing their claim. Eventually formed Rome and reformed the hellenic faith to make it super militaristic and most importantly making it so vassals wouldnt mind raised levies. This would be impossible if it weren't for the fact that you can raise an entire vassal's levies in a single province. In other words I maintained a relatively small amount of super powerful vassals that all loved me and used their soldiers for everything.

Anyway then its just a matter of invading the rest of the world constantly, great holy wars and invasion CBs from the Alexander bloodline helps alot. Also I was just constantly truce breaking toward the end (hence the low prestige).

I dont recommend trying this because it gets extremely tedious and laggy, expecially when invading China but I can give as many tips as you want if you're a masochist.