Step 1) Acquire a large standing army, logistics, and lots of rockets and big radars (the bigger the better, stealth planes are very tricky to detect without massive antennas). Step 2) Nuclear weapons, ICBMs, and stealthy submarines. Step 3) Sufficient infrastructure to support autarchy and the above two steps.
In the current age, without the patronage of a larger state, starting a country ex nihilo with the expectation that it will remain sovereign is not possible (surprisingly you don't hear from Thiel's pet think tanks about the sovereignty problem when it comes to building a country in the middle of the sea, you would think the founder of Palantir should have some creative solutions).
Modern assymetric war doctrine relies on air superiority. Deny other countries the skies and you will be free. You don't really need a conventional navy if you don't plan to project force. Countries like Iran use a large fleet of small boats armed with rockets to counter bigger navies (and more importantly, carriers. Their boat swarm strategy however is untested outside of simulations). A country is built on legitimate violence. At the end of the day, the nuclear triad is the only and ultimate guarantee of true territorial sovereignty. Legitimacy will come naturally when sovereignty becomes absolute.
Sovereignty in political science and international relations lie on a spectrum. Countries like Cuba have quite limited sovereignty[0]. A fully nuclear armed seasteading state may never achieve economic sovereignty due to sanctions by incumbents, but it will definitely have full perpetual control of its territories and in the current multipolar world, there are plenty of opportunities to increase economic sovereignty.
https://www.brookings.edu/research/cuba-the-u-s-and-the-conc...