First and foremost, time and effort. That's what it is. That's what I believe, that's what I see. It's time and effort.
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But for the answer you're lookin for, you could do something along the lines of coming up with a regiment (practice schedule) and actually following through on it. And I dont mean like "Oh ima sit here and play the game for 30 minutes, and ima turn it off).
I mean like -for fox for example-, you'll do something like
30 drill shines
30 SHDL
20 RSHDl
20 Shine-Grabs
50 Shine-B.airs
etc, etc, and you work till you do it.
You sit down till you do everything you want to do, then you test yourself to put yourself on the spot and make yourself get better.
*edit* kinda stopped before I should.
And as for as far down as you go for you practice, you start wherever you need to. If you can start with stuff like that, good stuff. If you need to go further down, do it.
If you need to work on actually L cancelling your d.airs, you throw that in there.
You need to just work on shorthopping, you do it.
The point is you do whatever it takes to get it down, for some people it's not much; for others it seems like it wont happen. I've never really thought of it as much of a "Can and Cant" thing as much as it is time and effort. I've never thought of learning fox as something that people literally cant do, the button timing isnt strict at all (except for like, JC shines), the problem is people dont want to actually sit down and work on pressing the buttons.