How do you define "shoot well"?
Depends on what you plan on shooting, doesn't it? Competition, self defense, meat, paper, smallbore, centerfire, smoothbore, handguns, airgun...? Each one has somewhat different requirements, equipment and expectations.
There is a load of great advice beginning with Pax's initial responding post, just as your questions are an indicator of some desire to ask the right questions on some quest towards better marksmanship.
Today, you eschew anyone recommending training classes. Someday... you say.
Good enough.
I was brought up shooting by my Dad and Uncles. They all learned in the military.
If you can decide WHAT you want to shoot, you'll have set your first goal. THEN you need to decide how to arrive at that goal.
There's a world of difference between being a poor kid with only a .22 and 5 rounds of ammo to put food on the family table or else go hungry (you don't know how many times I heard my Dad and his brothers tell me this old story of being a depression era poor Missouri family) and being a World Class Rifleman at Camp Perry.
So what's YOUR goal? I come back to your opening question I quoted above and ask you...
How do you define "shoot well"?
My Dad taught me the following goals:
For self defense, 5 rounds, 5 inches at 5 yards in 5 seconds.
Hit the deer 1st time lungs/heart, be ready for instant follow shot.
Shoot the 1st quail, not all of them.
Take your time when punching paper.
Have fun every time you shoot, but have a goal every time you shoot.
After that, everything else just kinda falls into place for me. I would still like to be as good a shot as my old man was.
Oh yeah... and take some classes.