don't use wiz kick nearly as much as you probably want. if you're not forcing them into a bad position, they're probably baiting it out.
don't use dair and fair as much as you probably want.
fair is a perfectly good spacing move, but fadeaway fair is easy for a lot of the cast to bait out, call you on it, then punish you either during startup or on a whiff. not saying, don't use it, but be mindful of why you want to use it and make sure that's a good situation for it. if you space properly and hit their shield, you can pretty much always at the very least roll away safely, and if you've forced them close to the edge you can start pressuring with spaced fair-f/dtilt or gfc or utilt or another fair or uair or etc...
dair gets overused by new players because it's really ****ing cool and it's a sexy move, but it has some good startup and a limited hitbox range (entirely below him, mainly) and since ganon doesn't move fast, you can't effectively extend it, like falcon could. further, since the hitboxes are all entirely below him, it's hard to make a safe spacing for it, due to lack of speed, so you can technically make it safe on shield with l cancelling, but it's easier to mess it up and you're putting yourself in a riskier position. save it for good read punishes and trying to edgeguard someone, as part of your overall repertoire.
for neutral, you're going to spend a lot of time grounded trying to poke out with ftilt (dtilt is better for comboes, but slightly slower and a bit less safe). once you start to figure them out, you can mix it up with rar-bair, which can be sh-auto cancelled, or delayed for a lower hit. with the ac, you can sh-bair-dj-[aerial] or float-[whatever] or waveland away or towards, with their own set of options out of them (sh-bair-wl away jump in fair works surprisingly well, for example). there's also sh-instant uair for a good coverage attack. since it covers so much above and around him and comes out frame 11, it's very useful against characters that want to approach from that 45 degree in front of you angle. hopefully they reinstate his hitboxes on nair, so you have more options to use, but until then, keep this more as a combo linking tool (it's frame 5 and does really good damage, so you can do stuff like dthrow-sh nair and if you get both hits you just dealt around 30 damage. brutal).
ganon's neutral is pretty weak, though. you can't force the opponent to do much and you're slow, even with wavelands, so you have to be patient and know how to play the stage control game. your neutral options rely mostly on the threat of your punishes and the fact that ganon can actually kill off of stray hits really well at what would be "just approaching" dangerous for a lot of the rest of the cast. to this end, as i said, you'll spend a lot of the neutral phase just trying to push people back towards the ledge so you can start forcing your options on them (think like a street fighter zangief playstyle. he's got bad footsie tools, but they get WAY better once he corners you and removes your ability to retreat and do bait-punish).
i'll find a link (or maybe someone else will) of the afc frame data. it's really rough, because there are inconsistencies with different di's out of it up to maybe 3 frames, but it lets you get a rough idea of what you can get guaranteed out of afc and whether or not you want that vs a hard read punish.
a lot of ganon's gameplan is centered around finally forcing the opponent to take an option, then punishing them for it. hence, the pushing around in neutral. you need to get good at finding patterns and preferences and exploiting those, especially any situations you have advantage (tech situations; staggered neutral, where you've gotten a hit but they're just in the air not being juggled yet; forcing them to shield near the ledge; things like that). if you can chaingrab that character, do it. if you can't, find out if you can do jank **** like dthrow-jab/grounded neutralb-regrab because they fell asleep on your something. if you can do something showy, to get inside their head, do it. the more advantages you can press on them the better.
and ganon is pretty bad under pressure. he has a few good options, but they either hit relatively high (nair, frame 10, hits near right below ganon's chest, i think; uair, frame 11, hits closer to his head; grab, lolz) or are really committal (upb oos, frame 11, can be turned to either direction at any point before it starts, has good grab range, but if you miss you're really high in the air...). for upb oos, specifically, don't be afraid to toss it out if the opponent is blatantly disrespecting you since it deals 18% and has pretty good kb on it. once they have to start putting more energy into thinking about their pressure and baits you get a bit more breathing room.
for practice, once you get used to the pm physics (since they're slightly different than melee's) the most useful things would be bair-wavelands and perfect ledge hop wavelands. other really useful tools to practice are rar-wavedash (for edgehogging) and learning to manipulate the float physics (so, breverse, reverse b, and wavebouncing) to open up your options out of it. also sh-uair-wl is nice, though more limited in application than sh-bair-wl. and practice sweetspotting with ganon. i don't know a good way to do it easily, but his sweetspot is really finnicky and seems to require only one small range of angles and distance. try to get used to that and practice it. get used to sweetspotting with sideb, as well.
i'm going to start off and say get really used to float. like, overuse it. get to where you're predictable about using it and are getting punished for it. then start cutting those bad situations out. use it to go super deep offstage and uair someone recovering, or just to wait out their offstage stall options before getting a tipman or bair or fair or whatever. in neutral, full jump float and watch your opponent. there's a lot of the cast that can just throw a random hitbox out and catch you, but anyone slightly slow can probably be reacted to if they jump in at you by dj-fair, and if nothing else it's a mixup allowing you to either go for sh-aerial, full jump-aerial, sh-float-[thing], fj-float-[thing], or skip the float and dj out of the first jump.
a small amount of the cast (slower characters without quick, big forward/upward coverage moves) can be sorta float camped by ganon. peach sits around this camp, so you can dd around to establish spacing and then sh float and react to whatever she does. if she retreats, you keep floating foward and just wait it out. if she approaches, you can dj out and aerial, or just dj out and reset the situation. if you force a retreat, you've gained stage positioning, if you're forced to pull back, you've likely just kept things neutral. as i said, though, not useful against too many characters. even ganon can just toss out uair and your dj won't get you out of the range ><
when you gain momentum, float becomes far more useful and way safer. if they're shielding you can now mix up your approaches with space aerials or jump-float spaced aerials (just simple timing mixups do a lot of work, especially when people are establishing patterns) or just tomahawks, or jump-afc (because they're just done with your ****ing aerial mixups and are just gonna shield and wait).
i'm not as good at this portion, but it's also great for tech situations, because you can, say, afc, then jump toward them and float. if they roll behind you, you can cancel out, wl back, and do a thing. if they go forward, you can just fair (or cancel into wl-thing, as well). or if they're forced to tech further out, say from bad di on a semi fastfaller eating fair, you can dash-sh-float and cover a lot of distance with it, while remaining relatively open as far as options go.
and, yeah, big long rambly write up. sorry, we don't have a good guide up, so this is probably the best i can do on a whim. feel free to ask questions, i'll see what i can do. check the vid thread and watch people critically, understand why they did what they did and think about it. it'll help you out as a player, too. and if you get any vids up, don't feel bad about asking for critique, that's how you start to find bad habits (and even your really good ones).