I do this as a side hustle. I trained by reading books, doing research on the internet, and practicing at home. Honestly, the return on the time investment isn’t great. However, I enjoyed the learning process enough that I’d do it even if there was no money in it. So it is a hobby to me. It just happens to make some money too.
As others said, you’ll need a good bankroll to play at a high enough level to make the time worth it. But if you have a day job too, your bankroll is considered replenishable. If you hit some bad luck and lose your whole bankroll, you are not out of the game because you can just save up some money again from your job. So it is ok to play with higher risk of ruin.
You need the technical skills that you’ll learn through practice and you need an appropriate bankroll. However, and more importantly, you need the right personality. You need to be someone who enjoys being in the casino, playing blackjack, chatting with others at the table, and not being too nervous about the heat you receive from the casino staff. If that sounds like you or you’re willing to learn, then great.