My suggestion is to search for open-source codebases in your favorite languages, study them, and start practicing them.

I'm definitely going to do that. I asked because I wanted to start off with some that I'd knew would be well designed. However, it would probably be more interesting to see a wide variety of quality than just the good ones.

Or maybe open source projects are all usually well designed. I haven't looked at many in depth. The only one I've really looked into was Clang to try to figure out why clang format ignored my style rules when styling files of certain names. (Turns out there is a list of file names that automatically are considered Objective-C rather than C++)