How to pronounce the LL letter.

Well, basically the development of Spanish LL to Y and ZH and SH is a continuation of a process Spanish has gone through for centuries.

For example the J in "hijo" was originally an LY sound ("filio") which developed into LL --> Y --> ZH --> SH --> KH.

The transition from SH to KH may seem weird, but this phenomenon can be observed live in the Swedish language, where the process is still going on.
For example the word for seven, "sju" has a pronunciation ranging from SH to KH.

In Spanish to pronounce LL like a palatal lateral aproximant sound is becoming more and more appreciated, despite most of people pronounce LL like Y, and like the English digraph SH in the particular case of Argentina. In America there is the same situation despite in Bolivia and Paraguay thy distinguish LL from Y.

people get a life