I can say, without undue hubris, that I'm good at my job. We can quibble endlessly about what that might mean, discuss how one might quantify drink preparation and customer service quality. We might even debate whether it's possible to do so. So given that, I can't prove I'm good at my job. But I can say so, and I will.
Being a self-professed good barista, I can now declare how one might join the ranks. I still can't tell you what skills are strictly necessary to receive one's imaginary certification, of course. In that regard, I'm taking the Supreme Court's ruling on pornography to heart: I can't say exactly what it is; but I know it when I see it.
So, having said that, I am willing to go out on one limb in particular: To say how one becomes a good barista. There are several steps, each vital to the process.
1. Find a coffee shop that's hiring. Apply. Take whatever shift you're offered. This will be closing. Sorry.
2. Do the job. At this point, you will mostly be preparing cold press and cleaning the espresso machine. You will forget to empty the pots. You will probably not make drinks. When you do, they will not be very good. At all. That's ok.
3. Continue doing the job. Your frustration at being mediocre will lead you to seek out online resources. You will find better ones than this. You might even seek out a talented barista at your place of business or a training school. These are viable options, but mostly, you need to develop your own feel.
4. Make lots of drinks. Then make more. Try every different technique you've seen or read about, then several more you just came up with. Eventually, some will feel right. Later, your drinks will become good. People who have only ever had Starbucks will be impressed.
5. Do this long enough that people begin to wonder when, if ever, you're going to get a real job. Start calling yourself a barista, rather than saying you work at a coffee shop. Be thin. Have stubble (if male). Talk about everything with everyone, always acting more interested and educated than you are. Pretend to care about issues, but mostly, don't.
6. Make more drinks. As many as you can. Do latte art, to impress the people who have had more than Starbucks. Talk at length about the quality of your crema and foam. Think that, maybe, you could compete. Realize that it's easier to talk about it than get crushed by more talented people. Keep talking about it anyway.
7. (Optional) Start a blog.