You already know the answer to this

• You have to use the language. A lot. To solve as diverse a range of problems as possible
• You have to read (and, ideally, work with) other peoples code, preferably ones who have a degree of mastery (i.e. a deep understanding of at least some aspect of the language). By extension, if you can talk to people about solving problems with the language then so much the better.

It comes down to hard work and determination and for all that you may never get there.

Think of where we get the term from - to become a Master Craftsman you had to be an apprentice and then a journeyman and only them might you have the chance to become a master. Along the way you would be taught and you would practice and you would observe and question others at their craft and (hopefully) learn from them.

There's no magic bullet...