Sorry, I'm not much help with that kind of advice. My knowledge about it mostly comes from dim memories of my time at university many years ago - with only some very minor bits being kept fresh by regular use.

My only suggestion is that it would be good to use a reputable textbook, so that you can be fairly confident that it is correct and not cutting corners; and then when you practice yourself, you should also make a conscious effort to not cut corners. (It's always tempting to just fudge any changes to arbitrary constants as being unimportant - since they are still arbitrary constants anyway; and mostly it doesn't matter. But that's a bad habit, because sometimes it does matter, and won't notice those times unless you've been paying attention all the time.) I won't try to recommend specific sources though, because I'm very out-of-date with that. I'll just say that published books used by universities are likely to be better than the notes of a private tutor or small online course.