I haven't learned, but I would guess that starting with a few letters, then using spaced repetition to add more and more. Since you usually hear, not read code, aurally is probably most useful. I'm sure there are apps for this now.

The good thing is the human brain is quite good at associating specific sounds with meanings, as this is how all spoken language works. So you're just learning some new sounds for letters, not a completely new way to represent information. I'd guess that part comes pretty quickly for most people.

Learning to tap out the letters on a key is probably a bit more challenging, as that requires some physical dexterity, but like playing a musical instrument, I'm sure most people can become at least adequate at it with some practice.