How do you get bankai?

Answer 1

You ether

A: Train your physical and mental bodies for hundreds of years at the soul academy, developing a relationship with your sword so great that it will lend you its power to achieve bankai..... OR

B: Buy it

Answer 2

By stabbing a doll and fighting with the physical manifestation of your sword (otherwise known as Zanpakutō)

Oh and also you have to have an out of body experience... Quite literally.

Answer 3

On character select hold w+j or in the game j+k.

P.S. Watch out for the glitch!