how does this all come about? In practical terms, how do you acquire the land, develop infrastructure, form a system of government, and convince regular people to join you?
If I was hired as a political consultant, I'd say that there is next to no chance to develop a new country. If it is succesful it becomes a threat that is easily defeated. If it is unsuccesful, well, what's the point?
Therefor it works better to work with existing infrastructure, land and government.
So step one would be to identify the country where you want to do it. Considering nativist movements have little interest in being led from abroad, it's going to either have to be the country you're in already, or if you're extraordinarily wealthy or powerful, but then you could hire better advisors than me and the point becomes moot again.
So the only two steps left in answering your questions is how to convince the people and how to reform government.
The government doesn't really need large reforms. You could have the same system with different goals and it would reshape laws over time in accordance.
How to convince the people? Ever played the game werewolf or mafia (it's the same game). About 1/4th of players are designated werewolf/mafia. The game plays in two phases, night and day. During the night everyone closes their eyes. Then the secretly chosen werewolfs open their eyes so that they know each other and choose one person to eat. Then during the day people get to talk and get to vote on who to lynch. After the lynching it is revealed what the role of that person was. This continues until there are only werewolfs or regular villagers left.
Now, once werewolfs are a majority, the game is done, because they control both night and day killing at that point and they don't lack information.
Demographics and democracy also kinda work like that.
In my experience playing, there are always some villagers that are complete idiots. They don't really understand the rules and their implications and they easily end up acting either randomly or in the interest of werewolfs. You need about 60% of people to act with sense. But 25% are actively working to undermine and werewolfs rarely make bad decisions (they have perfect info), you need to get the werewolfs under control before you lose 15%. Because they will hunt the best leaders/deducers at night.
I say this to show how hard the strategy is to persue. Once you organise a nativist movement, you hard-eliminate everyone outside it from supporting you politically. And you might not even get 50% of those inside it, particularly after 6 decades of oikophobic culture. Even Frozen II now has a theme of "my father betrayed and raped the noble savages, I have to repent in his name" and nativist heroes statues are being toppled across the west so it doesn't look like that is going away anytime soon.
The US is closing in on 50% fast so as a country it won't be able to support such a movement without widespread violence: but that again destroys infrastructure and gov rather than using existing structures.
European countries have more of a chance, but the people are politically and culturally less advanced. By that I mean that we have less capacity to think free and in our own interest compared to americans. There's also the pressure from the EU and neighbouring countries. The former in particular has no problem hurting both its own countries and yours in persuit of their destiny. They'll happily tank mutually beneficial trade to hurt you for not playing along. And since they have a huge hateboner for any country that isn't demographicly transforming, it's going to be hard.
That's why my advice wouldn't be to do it openly. You would have to become your own group of werewolves. Certain in your allegiance and ruthless to those outside it.
And you'd try to convince people simply by pointing at the direction things are going (oblivion) and what they could do to prevent it (follow you). You need to roughly get to 25% support to get a chance of success and hope the villagers don't notice in time to lynch you by day.
Once enough political power and will is attained starts the easier part. Immigration restriction. Remigration. Incentives. Segregated culture (same thing you see now with white people not being allowed to play poc, but vice versa being fine). It's just a slow march of getting people used to it at that point. Of course it's in the interest of the original host nations of the immigrants not to play ball and simultaneously destabilize you and keep their loyal subjects in your country (sending money to their families). However, that also gives you truthful propaganda that you can use to create more political will among those who should support you.
