The tl;dr version:
You just gotta practice it a ton. Put some time in just playing the game, and put some time in practicing specific stuff.
Longer version:
I remember back when controlling ZSS in brawl was hard, and now i laugh at the fact because I have (mostly) solid control over melee fox with all kinds of cool tech stuff like waveshines and multishines.
I'll give you my method tutorial style for learning short-hopping:
1. what button do you use to jump? Y or X?
1.5: if you use Y, can you shorthop 9/10 times?
If you answered
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but no to 1.5... well you're using
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from here on out.
2. Do you try to shorthop consistently by pressing in and quickly releasing the jump button?
If you answered yes to this, and your short hops are less consistent than 3 out of 4 tries (being new), try this method:
You've already switched to using
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right? Good good. Are you still trying to press and release quickly but not getting consistent results?
Try just
quickly grazing X with your thumb. If your right thumb naturally hovers over
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(ish), just bring it rightwards and downwards, so you basically barely press the X button, and because you graze over it quickly, it releases more consistently.
Hopefully this will help your SH consistency, although once you get this down the next step is making your fingers quick enough to follow up (now that your thumb is to the right of your controller, you gotta bring it back quick to hit A). Get the shorthop consistency down though - the speed will come with passive practice
Soon enough you'll be pillar shining
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Or fair string -> ken combo
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Or multishining
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Lol etc