The theoretical concepts you hold in your head are everything. And the more widely applicable they are, the better. Concepts that can be applied across dozens of disciplines are much better than concepts which apply only in a narrow circumstance.
This is an important point to understand: You cannot see something if you don't have a theoretical concept in your mind for it. For example, you cannot even recognize or observe evidence that the world is curved like a ball, if you don't have any concept of spheres or curvature already in mind. We cannot notice that for which we have no concepts to apply.
Some people have muddled ideas, which they are able to make work for them. But they aren't terribly good for you to learn from. For example, prior to and around the time of Galileo, many felt that the propensity of something to sink or float in water was at least in part because of how sharp it was. Knives sank because they cut the water. A log floated because it didn't have an edge to cut the water nearly so well. This idea kind of worked and helped explain observation. But it was quite muddled.
During a debate on this topic, Galileo had prepared for himself some wax and metal filings. He shaped it into a cone, as he added metal filings. Eventually, testing showed him that the wax cone was finally neutrally buoyant. At this point, he made a demonstration using a deep bowl of water. He placed the cone, point-first into the water very slowly and eventually released his hold. The cone floated. Then he removed and inverted the cone's orientation and lowered it into the water until only the tip remained, when he again released it. The cone sank. He then asked the audience to explain why, if the idea of "cutting water" was important to sinking and floating, that the cone floats with the blunt end down and the cone sinks with the sharp pointed end down.
Now, Galileo didn't muddle his question with the idea of surface tension. I'm not even sure he was aware of it when considering this demonstration. But that's not the point. The point here is that he was challenging an idea that others held and was able to come up with an experimental test to make the point.
This is the kind of sharp thinking about the world that you want to watch, study, and learn from. Not the confused ideas by people who don't attempt to challenge their ideas, but instead only argue for them without struggling to find a way to disprove what they believe.
Seek out the work product of the very best you can find (and I'd definitely recommend reading what you can grab from Galileo's writing -- he was even able to anticipate atomic bonds!) and learn to see as they see the world. Don't assume they are always right. Galileo's test above didn't account for surface tension, for example. (But that wasn't its point, either. He wasn't providing a theory, he was challenging a different one.) Most of what the best of us think about the world is probably wrong, anyway. It's not the conclusions you are after. It's how they think that you want to study and learn from and acquire, where possible.
You should also look for concepts which can be applied in a variety of circumstances, using different slices of the idea when facing different slices of the world around you. So a good teacher will usually express ideas in a variety of different perspectives in order to help convey to you the fuller notions -- which cannot be gained by only one slice or perspective but require multiple viewpoints in order to acquire.
I also think a good teacher will let you struggle for a while on your own. What's the point of just handing you the theory that this oddball concept we today call density, the mathematical ratio of mass (another oddball concept, different from weight) to volume, so that you know why things sink or float? If it is just given to you on a silver platter, how will you learn to think on your own?
A bad teacher, attempting to allow students to uncover this idea of density, might use a variety of objects that all have the same volume but different masses. Students playing with these, in water, would only discover that heavier things sink. They would not discover this special ratio I mentioned above. So there are very bad ways to teach. Be aware of these.
Find people who have been around the block on a subject and are willing to talk about how they see things. Find books that are well-recommended by such people. And use your own head, too. Sit down and apply what you learn so that it deepens inside of you and becomes a part of how you see.
Textbooks written earlier in a topic's development will usually do more explaining than more modern books, which focus on increasingly more narrow perspectives as they expand into minute details of continuing interest. So, I will often look towards earlier texts. An example is that if you want to understand compilers and operating systems, it is probably better to look to earlier textbooks (say, from the late 1960's to the early 1980's.) This is because they don't assume much about the reader and will take you for a gentle, longer ride. The best books on compilation (for me) came from DEC in their books on the PDP-8 and PDP-11. The best book I know of on operating system design comes from Douglas Comer's 1983 XINU book. (His later books are not as nice to read.)
That's about it, for now. I have to leave for a bit. Just thought I'd throw that out.
Another approach is to find someone who has written a paper on the topic and then write them. I've often found them quite willing to share, when someone expresses an interest in their field.