Chiefly, by becoming the owner of a ship. I’ll talk about this more in Part II of Vox Populi, Vox Deii, but the Ironborn have a strange social structure whereby a very restrictive warrior aristocracy and hyper-serfdom sits alongside an ideology of herrenvolk egalitarianism.
So even though the warrior caste of the Iron Islands is a very exclusive club which looks down on social climbers like the Codds, there’s this pretence that the lowliest fisherman is equal to the highest lord at the kingsmoot, etc.