The question How is Armament Haki obtained or learned? is closed as primarily opinion based, because of an incorrect assumption that primarily opinion based means "we don't know the answer yet".
Based on this meta post, questions whose answers are not yet revealed should be left open. Hence, I request that the question be reopened.
As I mention in the comments, there is a realistic chance of the answer being revealed in the future. A flashback showing Luffy's training at Rusukaina is one such possibility.
Note: In case anyone is wondering why I voted to close and then mention here that the reason is incorrect, that's because of the SE limitation of not being able to cast a reopen vote until the question is closed.