I'm a proponent of taking lessons from a good instructor. There are just too many good players who have benefitted from them. And just as a wise man once told me it wouldn't, "trying different things" almost never works.
Please bear with me on this, but what if lessons were not an option? What process or system of learning might be second-best?
Video would play an important part I would think, as would shot results. But you'd still have to know what to look for. Keys 1-3 are pretty easy for us non-pro's to evaluate. And ball flight might help identify the other two... correct?
What about angle of attack, wrist hinge... all the other moving parts? How would an amateur - on his or her own - identify the root cause and priority piece? And how would we know it's time to move on to the next?