After decades of successful and unsuccessful self study, the thing I have found for myself is that I have to have an end goal in mind of what I want to do with the knowledge. Then it's usually pretty obvious how to work backwards and figure out how to get there. I've been tremendously unsuccessful when trying to learn just with the goal of learning. It's much harder then to quantify what is good enough, and just end up with a very surface level understanding even after putting in a lot of work.
