What's the actual traditional, correct way of even becoming a guru?

You'll have to check, A, who is your guru? Who is the guru's guru? Whether he also was a proper Vaishnava, he followed the Vaishnava lifestyle, he had attained perfection in a sadhana or not, and then who was his guru. Three generations of gurus have to be very clear. If your guru has not attained siddhi of the deity, it's very unlikely that you will also get directly siddhi, eventually, of the mantra from that guru.

But then how are they qualified as a guru?

Processes involved. Also, number one, there are many processes involved which you will never know until the guru shows you. You cannot learn it from books.