1. Actually have an interest in history as oppose to just want to sound smart. I'm serious about this, if you are actually interested in something you learn it organically instead of forcing yourself to read through books/articles which is frankly not worth it unless you get paid. Find a period you are really interested in and pick up books along the way.

2. Read analytical books that goes deeper than just "X happened in year Y". Wikipedia is a good source for that but you want to read material which gives insight into what happened. So we know that there was an occupation of Japan in 1945, but a book like embracing defeat analyzes the effect it had on Japanese culture and post-war policies.

3. We are all busy (I end up doing a lot of readings on bus/commutes), so podcasts/youtube are sometimes really good. Something like (https://www.youtube.com/watch?v=1h9oOroEWzg) is a really good overview by a noted Roman scholar on the Roman monetary system. However, this sort of gem exist right alongside of more popular video about ancient aliens building the pyramids so discretion is recommended.

4. Participate in discussions and debates about the time periods you like, it challenges you find points to support your arguments and thus increase your knowledge, as well as learn from the other side.

The solution is that you should try to find good, peer reviewed, books and then search up their authors to see if they made any good audio material. There's all sorts of fascinating talks etc from academic conferences on there (usually with a low number of views) if you are to look for it.

More specifically, do I have to obtain a Master's or PhD to have that level?

I did my bachelors in Computer Science and Masters in Economics so no.

Can I just say that just because I have flair doesn't mean I'm at a flaired level in the sense that there's not a set strict criteria a flaired user has to adhere to on this sub (apart from three quality answers and even then there's quite a bit of variety) compared to a subreddit like /r/AskScience where you have to have a degree to apply. There's a load of flaired users here from a variety of backgrounds not all of them (myself included) are history-based.

I don't even have any form of degree, whatsoever. I'd read up on historiography of your chosen speciality though, as trends can & will change for example (using psychological studies) Milgram. S (1963) which looked at how people far people would obey orders. Burger. J (2009) looked at whether Milgram's research is still valid today, would they still obey today?

For a more historical look on changing trends in history, check out /u/commiespaceinvader's sub podcast episode on the "Intentionalism vs Functionalism debate in Holocaust studies"

If you are really interested in expanding knowledge on a specific subject in addition to all the things other have posted, I think the most important thing if you are not getting paid for it or make it your life choice to be a historian of that subject (and what a life choice that can be I say as the Nazi historian) is to have fun with it through a genuine interest.

I look at being a historian of a certain subject as pretty much imagining yourself as being a detective. You want to know about your case and you start by picking something that interests you and start reading up on it. The really essential thing is that it should be fun and interesting. And when you have that, it is time to embrace a lot of the tips given here, such as recognizing and embracing bias (that ones really important) and getting into articles and the arguments surrounding that subject.

It'll take time but the most important thing is that it's not like a second job but like a passion.

Read. A lot. Then read some more. Get to the point that you're very familiar with the facts at hand and start to focus more on the interpretation an author puts forth. Textbooks are handy starting places, as are the Major Problems in ______ History books.

Once you have a firm grasp of most of the issues surrounding whichever topic interests you, find out if any library to which you have access (school, local, local university, whatever) has access to JSTOR. Search it for articles on your topic of interest. Start with the scholars cited in the textbook notes and the Major Problems series, then branch out by seeing who those authors cite, and so on.

Most historians (in the US context, anyway) use articles to flesh-out book projects and have some pretty fun arguments. Articles often contain more argument than timeline stuff. If your interests lie in American history, see if you can arrange access to Reviews in American History. The essays contained therein are very valuable in that they usually contain reviews of multiple books on the same or very similar subjects. There are likely similar journals for other fields, but that's beyond my ken. Get to know the person who runs the reference desk as they will often very cheerfully provide excellent direction.

Book are great, but in terms of coming to grips with 'the state of the field,' they're far less efficient. In grad school, it's more or less assumed that you have a grasp of the facts, so you learn to process books for their argument rather than read them page-by-page. Of the thousands of books I 'read' in grad school, I probably only read every word in a couple dozen -- maybe fewer. Remember that each book on a subject is in conversation with all other books on the subject. They will each emphasize different stuff, reach different conclusions, and offer some sort of criticism of each previous work. Read the introductions very well. Read the conclusions even more carefully.

Learn to be efficient in book selection. Avoid monographs until you're really mining the subject. Look for big syntheses that cover sweeping topics well (and which usually have excellent historiographical essays at the end -- these are some of the most valuable things you can read.) In general, if it's not from a University Press, don't bother. In the US context, the Oxford History of the United States is a good place to look for very good syntheses.

Learn to embrace and understand biases. The search for unbiased history is a sad product of our NCLB-laden times. There is no 'correct' answer, no correct bubble to fill in on the scantron form -- only answers that fit a particular author's life experiences. Understand those biases and why they shape an interpretation of a particular set of events. Weigh those interpretations against your own. Skim thru Peter Novick's That Noble Dream for a good, if somewhat polemical take on this.