system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did individuals become royalty and start a legacy?

How did an individual be determined as king?  What was the process used to make or determine someone as royal?
gpt
The basic recipe to becoming a monarch is military power plus succession.

In terms of growth, they start off convincing a bunch of their mates to go intimidate and kill enough of their neighbors that everyone around agrees to go along with whatever they wanted.  Then they use that power to round up a bunch of soldiers and go threaten to kill a bunch of people from the neighboring village until they go along with the whole scheme as well.  Repeat this over and over, conquering more areas, forging alliances with other leaders, and so forth, and eventually they end up commanding a significant amount of military power.

All that can't usually be done in one lifetime (though sometimes it can), so you also need succession.  In other words, when they die, someone needs to take over their position and have everyone agree that the new person gets to have all the power the old leader accumulated.  This is frequently done through children, but sometimes an advisor or someone similar will end up in the role instead (whether by plan or by treachery).  In any case, this means that the power that's been seized doesn't disappear when the leader dies.

Once all that is sorted, then they just need an appropriate opportunity.  Sometimes they just have a pretty big area under their control and declare themselves king.  Sometimes a kingdom nearby is weak, so they march over there, defeat the king's forces, and claim the throne.  Sometimes the kingdom they're nominally a part of has a monarch that dies childless or with only weak children, and so they march their forces over to the palace and tell everyone that they're king now and anyone who disagrees can talk to the big men with the swords.  Sometimes they're part of a democracy or republic or something but they think that's dumb, so they march into the capital and tell everyone that they'd all be better off (ie, not dead) if they switch to a monarchy instead.

No matter how you slice it, though, it pretty much always boils down to having enough people who are willing to kill anyone who disagrees that they're the monarch that nobody disagrees.