that's the neat part you don't.

but for real check your infamy, the higher it is the higher the percentage of radicals will be in newly conquered territory. if the primary culture of the state is discriminated in your country, then they get hit with the discrimination penalty which will reduce their wages which lowers their SoL thus more radicals. so try to either conquer somewhere where you won't get hit for discrimination, or change your laws to accept more people.

aside from that there's probably nothing else you can do but to watch the secession counter go up and crush them once they rise up, then incorporate them so they get less radical by your police and home affairs institutions, and your healthcare for easy SoL boost.

violent suppression decree also does not decrease the amount of radicals as far as i know, it just reduces the debuffs.

wait for incorporation, use decree for building and built stuff to reduce unemployement. also if you have giant delta in sol you are basically bonned (i.e. taking some gold producing country with 18 sol, and you have like 12, everyone is getting fucked on conquest)

You let John Brown command the army

Avoiding secessions is very hard. As others have mentioned, multiculturalism and low infamy help. As will religious secularism, high welfare pay, and public healthcare, to avoid more discrimination and rapidly increase quality of life. You're also right that buildings will help, but often not soon enough. Consider subsidising local industries to avoid unemployment.

In my experience, the most effective tool to avoid secession is using authority for local edicts - violent suppression, social mobility and emergency welfare especially. With those, I have seen the secession progress bar reverse course.

Tldr spend a lot of time and money and effort in it, or just post soldiers nearby.

Agreed, I don't think you really have enough time to stop it. The trigger is having over 50% turmoil and 33% population belonging to the ethnicity in question. I think going multiculturalism helps reduce turmoil because there'll be no discrimination against the newly-conquered pops, but it probably won't make that much of a difference. I don't remember if having cops or using the decree for suppression reduces turmoil itself or just the effects, but those are another option.

I think higher Sol, less discrimination and maybe settler colonization instead of exploitative help.

Othwerwise dont colonize, attack countries to ban slavery and make them puppets might work better at the cost of infamy.

This is what I'm talking abt when I say videogames are antisocial and reactionary. Look at all the suggestions here on how best to subjugate people in Imperialism Simulator 3000 from people who are anticolonial leftists. Would you play Victoria III if it included images of all the charred corpses and starving masses your actions create? Stop allowing yourselves to be sucked into a skinner box designed to normalize evil.

just kill them and then it end