Lots of ways to learn. Aside from what others have already mentioned, work on a lot of existing code. This can be others' code, or code you wrote yourself months or years ago. Whether you're fixing a bug, or adding an enhancement, or just trying to clean it up and make it better, stop and ask yourself occasionally: What made this difficult to understand or modify? How could it have been organized better? The other sources (books, courses, etc.) provide starting points for answering those questions but you need to apply that knowledge to real code, to test it and learn where it does and does not apply.