Read up on the Koch method. To be exact: Koch method with Farnsworth timing. You'll have to keep the letters in a certain speed so your brain does not have enough time to decode dots and dashes. So only the length between the letters is extended to lower the difficulty. The idea is that the lowest unit of comprehension are letters. After training you'll hear a -.- and don't even think of it as dots and dashes, it's just a K obviously.

You start just distinguishing two letters (usually K and M). I would recommend using a trainer app that gives you a string of letters by sound, and to write them down on paper. Then you can check how many you got correct. Do not try to replay letters multiple times, just skip it and move on. If your result is better than 95% or so you can continue and introduce a third letter, and so on.

There is a certain order of letters considered more or less canonical for use in the Koch method which you can look up.

Edit: Back then I learnt it with https://lcwo.net/, it's distraction-free and quite nice.

As another reply said, the consensus I read is that you really shouldn't memorize the table of "-.- is K", if you do you'll likely need to "unlearn" it to get any level of speed. The best thing is not really think about the dots and dashes but train on the sound; "dah-di-dah" = K.

It's a lot like how when you read you almost never read every letter, you look at the entire word and that turns into an idea in your mind.

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.