How is Armament Haki obtained or learned? Armament Haki is being used by a number of users, namely Luffy, Sabo, Garp, Chinjao, etc.

So, has there been any explanation given about the process or training involved in obtaining this Haki ?
As an introduction I will quote the wiki page on Haki:


  Haki is a mysterious power that is found in every living being in the world. It is not that different from the normal senses. However, most people do not notice it or fail to awaken it. Broadly, there are two types of Haki common to everyone, given the proper training


As mentioned here, everybody has the ability to learn Haki, given the proper training. We have seen lots of examples of people who had obtained Haki through training, like the girls from the Kuja tribe, Luffy during his training with Rayleigh, Cobi during his training with Garp, etc. Almost every person we have seen, that uses Haki has gone through some kind of training from a Haki expert. The problem is that none of these training sessions have been shown as of writing.

Training is not the only way to obtain Haki though. As mentioned in the introduction, Haki is not different from your normal senses and that means that you can also obtain Haki subconsciously. We have seen a handful of cases, where people had obtained Haki on their own, without any training whatsoever. Examples of this are Ace and Luffy obtaining Conqueror's Haki, Cobi and Usopp obtaining Observation Haki or Sai obtaining Armament Haki. So it does not mean you have to take training per se to obtain Haki, but it is well advised to take training to fully control your power. Ace for example was shown to have Conqueror's Haki, but it did not look like he ever learned how to control it properly.