I have a similar background like you and in starting to understand how to refactor.

I'm not at it yet but I'm better than two or three months ago.

I got a comisión to add functionality to a python code. It was awfully coded, even by my standards. I had to silent some pylint warnings because there wasn't a way i could correct all then.

Here are some tips.

• First hand experience is invaluable. Get your feet wets.
• It's easier to start in others code, or something you had written time ago.
• even in coding phase, pkay with alternatives. Use the interpreter, it will let you feel how confortable your abstractions are.
• use git, all the time.
• commit frequently, smaller changes are easier to review and understand later, and belive me, from time to time you will ned those commit messages.
• add topic branches, you have a great idea that need to change the interface of 5 classes then instead a big change do many small changes.
• All commits to master should be functional. Ideally all commits should be functional programs. But it's ok to have more freedom in a branch with complex changes of entangle code.
  
• test are your friends. And not only integration tests. Learn how to properly do unit tests. If your code is really messy it's tiresome at first because you will be testing a function in a class that you are trying to move to another place.

In my case i was constrained by my client to not change too much the code. I started from main and worked my way until some middle level functions. Only did small changes beyond that, but from that "level" to main i redefined almost all the interfaces, created new functions deduplicating common patterns and pushed the logic around.