This achievement is easier if you start big because it's the total number of provinces your country has that count, not your subjects. When you hit 100 or more provinces without local autonomy or unrest. it will pop. You may have provinces with unrest or autonomy but if you have they don't count towards the 100.
The best way of having a low unrest is high stability and low war exhaustion.
Other things to lower it is to have an army in the province (-0.25 for each unit)
Convert to your religion (will increase it during conversion but end with lower value)
Convert none accepted Culture (may insult neighboring countries with that culture and cost diplomacy)
Separatism will go away with time, but the humanist idea group will cut that time in half.
local autonomy will only go away completely in your main land, any oversea province will always have at least 75% autonomy. so it's mostly about owning the provinces as cores long enough. Oversea provinces are all provinces that isn't in Europe and isn't directly connected to your country capital by land.
So France isn't overseas for England or the other way around. However colonies in America or Africa is.
I personally did this as Ottomans and this is actually just a milestone towards some of the other harder achievements.