I feel like if I don't know why they chose to do a specific thing, I won't know how to properly alter it.
My comment isn't meant to devalue knowledge but to put it in relation to "something else". That something else is the thing you have knowledge about. The thing to appreciate is that you can become an expert on knowledge itself, without ever becoming expert in the thing the knowledge is about.
Consider some painting theory topics: color theory, contrast, perspective, proportion, etc. Imagine someone who attains expert level knowledge of any one (or combination) of these subjects but they are still unable to draw a picture that is a pleasing representation of their subject. You can easily study all of these topics for a lifetime without every picking up a pencil or a brush.
My other comment mentions the map vs. territory distinction. So let's deeply consider this. You are in unfamiliar territory and you feel lost. You think to yourself: "If I only had a map then I wouldn't feel so lost". But does that mean you should spend the rest of your life studying map making? An alternative is to survey the territory with your own eyes and learn to pick out the trails that many others have cut into the wild. And then follow some of those trails. You might end up at a dead end and have to turn back to a previous fork in the road. You may find yourself scratched up as you try to get through dense thickets, or bogged down up to your shins in a swamp. Those are the kinds of experiences that teach you the land in a way no map could ever. And they are experiences you can't get by sitting in a tent studying a map. If your goal is to find a new trail through the territory - no map will even show it. That will only come from the hard won experience of trekking relentlessly through the wild.
As the philosopher Mike Tyson once said: "Everyone has a plan until they get punched in the face". It sucks getting punched in the face by other people, by learning a new language, learning to paint, learning music. Some people avoid it at all costs, thinking that studying the theory is the same thing.
Just remember that all of the scratches and bruises you are getting as you fail at painting are scratches on your ego. It can take it. You will get better, as long as you keep trying it is inevitable you will learn. And it is very useful to glance at a map now and again. Just don't get too reliant on it.
There is no a 'proper' way to do it! It goes like this, you change it, if it looks good, well, good job, if not, ask yourself why, and try your best to take that into account next time.
It's by doing millions of small mistakes that you improve. The teacher teaches one way that is kinda easy to grasp, it's not the only way and far from unique.
The further you go, the further you will see the same mistakes, and you will start to think in terms of volumes, shapes, shadows, perspective, even anatomy if you still struggle on some human body parts
Note that recipes are actually useful, it's just that they are not theory.
> "what if I want to change something?" It's indeed an essential step of learning and creating
I'm not slowly foraying into doing more original things which seems to be going loads better than before I did the copy-work. Currently I'm practicing drawing hands, and while it's not like I'm producing good sketches of hands, I can now totally see the path from where I am to good sketches of hands.
I'll share a sequence of sketches so you can see what a study looks like for me. This started off with trying to draw a specific characters hand in a specific pose, but turned into more of a "well how about figuring out hands in general first" thing:
0. I started with copy work, decent results, did a couple of these: https://i.imgur.com/3Fb3kDY.jpg
1. I moved on to trying to do a hand using no refs, somewhat predictable results, doesn't look like anything: https://i.imgur.com/wvgm1F3.jpg
2. After a bunch of repeating the same exact drawing it's starting to look like a thing: https://i.imgur.com/oMEVhOM.jpg
3. Took a break on hands, did a bunch more copy work of complete characters etc. Got slightly better at confident lines.
4. Returned to hands, switched to using my own hand for reference. Ignore the missing finger, I misjudged the spacing but pressed on regardless. You can clearly see the improvement from when I started: https://i.imgur.com/fZduLG1.jpg
5. Where I am today, probably going to need to practice different angles, but it'll be basically more of the same. Keep doing copy-work, Keep doing studies.
Important part of studies is to draw the same thing multiple times I feel. Do a sketch, try to avoid hating yourself for it being bad too much, pick one thing to improve. E.g. for the study sketches linked I've picked "try perspective" as improvement, but "get the right number of fingers" or "make the fingers the right length" are equally valid goals.
Notably absent from my workflow is tutorials or art-books. I'm sure those will come in handy once I've got the basics down, but for now they are just frustrating non-help. I'm making significant progress week-over-week still.
tl;dr: Copy work and studies is what works for me. Copy-work gets the strokes into your muscle memory. Studies helps you get a feel for how and why to place strokes.