The same way you build mastery in anything: practice.

Write lots of Python (or whatever language you want to be proficient in) code. Every so often, go back to what you wrote before and review it.

Read lots of Python code. Tinker with it, add features, change things, see how what you do affects the program.

And study -- read lots about coding in Python. Interact with Python devs. That sort of thing.

The following links have helped me upgrade my python skills. Also i do spaced repetition with new python skills i have acquired which has done wonders to retaining new concepts. * https://www.pythonmorsels.com/ * https://github.com/norvig/pytudes