So self-ownership says, I am sovereign over my own self. Nobody else has a greater right to make my choices than me. Basically because my brain is literally attached to my body. I am the moral agent of its actions.
It follows, then, if no one else has agency over me, then I do not have agency over them either. That is how we arrive at non-aggression.
I may not initiate coercion over another person to take away their agency and substitute it with my own.
Self-defense says if either one of us does try to violate that personal boundary, the other one of us justified in refusing, resisting, or repelling by force if necessary.
Individual rights are then arrived from that basis. My freedom of speech is based on the idea that my voice is connected to my brain, I have agency over speaking with it, no other human may control my speech but me ... Same with all the others. They are several steps away from the first principle of self-ownership but all still very closely linked, logically. All are implied merely by the fact of existence. This is why they are called "natural" rights. They are by virtue of nature .... our very being.
Then it follows, if those things are true at the individual level, they are also true at the sociopolitical level. Groups of people can't ethically do anything that they could not as individuals do. We may not vote away another person's right. Majorities may not coerce the minority just because there are more of them.
And so .......... We arrive at libertarianism as a set of policies that make individual liberty possible at the level of large groups of people. We have a framework for ethical relations that preserve the rights of the people.
If another group of people cross our political boundaries and initiate force on us, we may then defend ourselves against that attack. But if we were to initiate the attack on them, we would be wrong in doing so and they would be justified in self defense.