SO yeah, what are good ways to practice teching in this game?

I feel like the best stage for this might be Hylian skies and twinlight town (cause of the walls), but idk what items to use and stuff tbh.

Same thing with DI/SDI.

Honestly, I would just say play against a CPU. So long as you don't intentionally abuse certain aspects of their AI, you'll find plenty of opportunities where you'll need to tech. Just do that, and it will be better practice than trying to explode yourself into walls or w/e because it will replicate being in a match and needing to tech.

As for DI, again you can get that off of CPUs, but this one is really hard to practice outside of an actual match (I worry that trying to set up a DI test will just create bad habits). As long as you make a constant effort, this will come in time. I believe that DI is really hard for less experienced players because in order to have good DI you either A) Need to have great reaction time or B) Expect to get hit. Newer players DO NOT have that kind of reaction time (and in truth, most people don't anyways), and most players when they start are not expecting to get hit.

Not only will you gradually get used to getting hit and therefore become more accustomed to DI'ing, you'll also learn the best ways to DI each move and have a good idea of when an opponent is going to use that move.

As for SDI, that one is fairly difficult to practice mid-match (tho eventually you will need to do it in a game), so I suggest you just go into training and throw Smart Bombs at the ground and SI the explosion. Or use a second set of controls, put BM as P2, have him charge Stop fully and then use it on you. That move also has a lot of hitlag, so you'll be able to practice Smash DI on that.

Your first application of SDI in-game is likely to be on a multihitting move like a jab loop, but eventually you will be able to SDI Fox dairs as soon as you recognize that you are getting hit. In fact, regarding that example, if you find that you are fighting a Fox that really likes to use dair, you can pre-emptively start Smash DI'ing if you know you are going to get hit by him because you know he's just going to down air you, right? The hardest application of SDI is using it on INDIVIDUAL moves to help escape the combo or just to mess up your opponent's spacing on the combo. This is really difficult, so don't fret about it.

I'm trying to recognize needing to SDI a move like Fox dair if my opponent is using it a lot (or at least just when they use it, it's not a short move), but I do make a conscious effort to SDI during a combo to try and get out of it. That said, I really only do that against Naruto lol