How so? Well, most of us are learning the guitar because we want to, not because someone else thinks it's a good idea (ie intrinsic rather than extrinsic motivation). And we like to know exactly why we should do something a particular way. In fact my guitar teacher is probably pretty fed up with me asking him why I should bother to do it his way, rather than mine. The fact that he's pretty much always right (at least so far as guitar stuff goes) is irrelevant: as adults just don't take kindly to being told "it's just the way it's done", and are quite likely to ignore the advice unless we can see the reason for doing it or because our way isn't working 😒 (problem based learning). We are also likely to use many different sources of information apart from our teacher - reading around the topic, consulting web videos, guitar forums etc. as well as our own past experiences.
In short, as adults we like to be in charge of our own learning. So we shouldn't really be surprised that self-directed learning is at the heart of modern teaching theory, and as adults we may be able to use it to help ourselves in our practice. Even if we have a great teacher, she isn't there all the time so most of our "learning" is what we teach ourselves while we are practicing.
One of the essential tenets of teaching deliberate practice is establishing concrete goals, that once reached, represent mastery over that skill. We're not talking being able to play Concierto de Aranjuez here. Just small goals, specific problems. So suppose the problem is missing a string every time in a particular measure perhaps after an awkward shift. The first step is establishing the goal, i.e. making that shift at tempo in a legato manner (obviously without missing the string). The next step is to recreate the problem, breaking things down and analyzing them with a view to coming up with potential solutions. What is happening before we miss the string? Perhaps the arm didn't get into position because the elbow didn't move first. Perhaps it's hard to hit the string unless we pre-place the finger. Perhaps it's not something our hands can easily manage and we should think about a different fingering or position. Perhaps it's actually the right hand that's confused and causing the problem. Most of us have been told these things multiple times in different contexts, but perhaps don't think to use them to troubleshoot on our own. Drumroll: enter deliberate practice! After having come up with a bunch of potential solutions, the next step is to test to see if any of them fix the problem. If a teacher had been present, she might have short circuited the process and prescribed the solution, reiterated the key points necessary to "get it right" (perhaps it's pre-placing the finger) then asked you to do the shift slowly with the new technique. She might have interrupted the "wrong" technique, instructed us to go more slowly, whatever, until the correct movement sequence was firmly established. If we are teaching ourselves, obviously we may have to try a bunch of potential solutions and be particularly meticulous about checking we aren't falling back into the previous unsuccessful movement pattern, but the process is essentially the same. It may take longer than if the teacher was present, but as Lao Tse said: "If you tell me, I will listen. if you show me, I will see. But if you let me experience, I will learn."
