For a lot of time, my basic study was get some video on YouTube and watch it making some notes, and try to implement what I've learned in future games. But I did not use to review my games. In fact, even today the review part is quite deficient for me, mainly because I don't have too much time available.
However this approach lacks a very important aspect of studying: organization. That's why I like to follow a course, because I can make a schedule and keep my progress. But still there is that feeling of something is missing. Just watching is not enough. It is necessary to review hands to reduce future mistakes. And playing is also a big part! Accepting that some sessions are "investments", in which we can suck and make mistakes, in order to correct them and learn the right way.
These videos made by Doug Polk are one of the best free videos I've ever seen, and he states how to improve today, and be surprised: he does not recommend any course, video or book about it! Mastering poker and analyzing decisions is a
hard work process. Ok, maybe some solvers can give you the answer quite fast, but do you know the reason behind that decision? There are no shortcuts to understand poker. I really recommend watching it and start doing what he teaches at lease once a week in a review session.
Do you want to learn poker? Less solvers and more notepad + calculator + equilab. Enjoy!