Here's some stuff I've written up on learning the fox- specific tech. Hope it can help you.
Waveshining:
In my opinion the most important thing you can learn after all the basic tech like L-cancel, SHDL is Waveshining. It makes your punish game significantly better and is also very useful when pressureing shields.
First of all MAKE SURE you are practising this to always be hitting a character or shield because if you learn to wavedash too quickly (ignoring hitlag or whatever) it is super hard to unlearn (at least in my experience).
The way I worked on it was by first just shining and wavedashing out (on a character or shield of course). Practise on characters with high-ish traction like peach and falcon with grabs, up-smash and d-smash with out a dash. On characters like Marth, you need to do a dash after, then a JC grab or up-smash. I recommend for grab and u-smash on high traction characters doing the JC anyways because it only costs you a couple of frames and you have lots of time on high traction characters anyways (no harm getting in the good habit).
Once you have that down, the next step is working on waveshine-waveshine-waveshine etc. Start on Peach or Zelda, who have the highest traction do a waveshine on them, then another waveshine right after. You want to do the second shine fairly quickly during the wavedash. I say to wavedash out of the second shine because then when you work on several waveshines in a row it's easier because you're not stuck in shine.
Once you have the second down, work on getting a third, fourth etc. Once you can do this easily, work on ending the chain with u-smash, grab and D-smash.
Then move on to Falcon, who has a bit less traction, doing the same things. Go back to Peach/ Zelda every so often too.
Eventually, you should move on to Sheik, the hardest top tier character to waveshine who you can actually waveshine. (With Marth he goes to far and you the need just JC grab/u-smash him with a dash, and for fox/ falco/ puff, you need to thunders' combo, which I'll get to in a next).
On Fox, Falco and Jigglypuff, you need to perform a "thunders' combo", since they are knocked down by your shine. It's basically jab resetting them out of you waveshine. It's pretty tricky. FWI, the timing is that you need to hit them after they first hit the ground but before they hit it the second time after they bounce. If you notice that they are consistently teching your shines, you basically just need to tech chase them because you can't jab reset them.
In terms of choosing which option to pick after you string of waveshines, u-throw u-air does more damge than a u-smash if you can land it on the character at that percent, espeacially if you get multiple u-airs. At kill percents you should always just u-smash regardless because it's more reliable. D-smash is if you waveshine someone to the edge of the stage and think you can use d-smash to gimp them (and u-smash won't kill). This is especially useful on Captain Falcon.
Multishines, Shortens:
. For practising multishines, first work on "jumpshines" jump then B with the multishine timing. This is to make it easier to learn future steps. I strongly recommend NOT doing shine-jump-shine at first, but rather the "jumpshines" I've described (I originally learned this from Westballz saying it).
. Once you can do that consistently, you should start doing Shine-Jump-Shine-Jump. It is important to do the jump after because you'll aways want to jump out of multishines, whether to do a wavehine or more multishines or a JC grab after, and I found that for me it was hard to learn to not just sit in shine after the second shine. After you get this, start doing wavedashes, JC grabs etc. out of the doubleshines, not just the jump.
. After this, move on to doing more and more multishines!
. For side-B shortens, you can't really practise them badly, my only advice is:
-Practise them on the ground, because it is the same timing but this stops you from getting your timing for the second B press from a jump you might potentially do to get in the air.
-If you want to sort of give yourself a situation where there's a bit of "pressure", you can stand facing the edge so that if you miss a shorten you'll SD (like I mean in practise).
-Try also putting yourself in a situation where you would use a shorten, like a bit above the edge but pretty close to the stage and do it from there. If you have 20XX, you can use the save/ load state to do it repeatedly, without have to jump off stage (which could potentially lead to the timing coming off the jump like I said earlier).
