The funniest thing ever is that you can quite literally just... declare sovereignty.
If it's either publicly owned land or your land you can just pick it and decide "yeah this is the sovereign nation that i rule over now" as long as everyone on that land agrees then yeah you're a sovereign nation.
Like what're they gonna do? Arrest you? They can't, you're a sovereign nation, you don't go by their rules anymore, you go by your rules.