All of us try to learn, make us to understand and have a particular idea to remember and recollect the topics in a subject studied. How to become expert in a subject?
I second to Irfan Ali Abro . Yes one can become expert by reading extensively and teaching what one has understood. Writing down the understanding from writing can also help you to become an expert.
According to my experiences I just need 2 years concentration and focusing on something to become an expert in that field but the important thing here is you mustn't focus on anything else.
Do one thing with a real good concentration
Do other things less than normal (weak-->normal-->good way--> high quality with a real good concentration (that parenthesis is just for showing what I meant and talking about the quality of doing a job or anything else) )
In the other hand you need to improve your other skills like reading, writing ... and other skills like visual skills and.... . (I asked scientists about how to improve our reading skill and speed up our learning process you can check it here https://www.researchgate.net/post/How_to_speed_up_our_reading_skills_and_learning_process?_ec=topicPostOverviewAuthoredQuestions&_sg=O-Z5gvXE4I3OQxrVpvzl9LQWUfG9l8vZwr6XFgXWjMqJPgdX7zUnq11eZaJwyynflV-JgzblAwCwwmml)
It is very important question. Firstly, to be an expert in subject or specialization field, you should read many books and research paper in this field.Follow-up on everything new in the field of specialization, self-learning, continuous learning, and continuous professional development. All are important factors to be an expert in your specialty.