In Crusader Kings 3, when a county in your realm does not share the same culture as you, it can be difficult to maintain control. You’re probably dealing with a few members of the population attempting to rise against you. You don’t receive as many levies as possible, and they don’t offer as many taxes. The population has quite a bit more to provide you with, and you could maximize your rewards. To do this, you need to convert the people to your culture.
Your steward’s job is to convert a population in a county’s culture if it differs from yours. The steward is the one traditionally looking over your realm’s finances and your infrastructure. To assign them to convert a county’s culture, go to your council management page and find your steward on the right side, in the middle. The task can task several years, and it usually helps to have your steward’s Stewardship level above 10. Feel free to assign a new councilmember to the job if you can find another better suited for the role.
Once you have them working on the task, it’s only a manner of time. It can take several years for them to complete it. But when they do, the population should be more manageable for your ruler to control so long as they remain in the same culture. Suppose your Steward has a higher Stewardship score. In that case, random tasks and events might occur that increase the county’s culture conversion or take longer if they have a lower score.