system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If you were stumble across an undiscovered island, what would you have to do to make it your own country?
gpt
Go to the island. Set up what you need to defend it from anyone else who would want to take it from you, which probably involves cannons and missiles.  Declare that you are a country to other countries and hope that they agree.

That is pretty much it. What areas are countries is up to who recognizes them.