Edge guarding an opponent recovering high is a matter of setting up Bowser to intersect with their trajectory, optimally with a bair. There's a whole little metagame regarding if/when they pull the trigger on the air-dodge vs when you bair them. I like setting up Bowser just below them and a little closer to the stage with my first jump, then jump into my bair so as to make it look a little less telegraphed.
An opponent recovering low can be met with a runoff fair. It's really easy, literally run off the stage and press A. Don't even take your thumb off the left stick. If they're going straight up you can attempt to run off into a bair. Use the c-stick for bair so you can keep drifting right/left to set it up right, that's what it's there for. One note - if you do a runoff fair and they air-dodge towards the stage (which is to say, they air-dodge), it's not over yet - you can still catch them by using your air jump straight into a bair for a nice stage spike. You may have to mash B a little when you aerial fortress to the ledge, but this is generally a safe bread and butter setup. Generally either you hit them with the fair or the bair, or they manage to air-dodge both. Of course at higher percents it's possible to get hit by something, but many characters can't afford to throw out an attack after they've just had to use their second jump. Their first priority will be getting to the ledge, making offstage play moderately safe.
In terms of dunking recovering opponents, it's also easy - the opponent just needs to start recovering in a way you notice is exploitable. For some reason, sometimes opponents would rather air-dodge at a height just above Bowser as they return to the stage rather than going to the ledge. Sometimes minus the air-dodge - maybe they think an aerial will save them, maybe they're holding off on the air-dodge. Right as they are about to horizontally get above the stage, you (standing at the edge) jump and dair. Don't do a full jump, think of the dair like something you're buffering out of the jump, like an autocanceled short hop fair would be. Only do a full jump if they're coming in extra high. The rising part of dair will make for the extra height. These sorts of height judgment calls will become very certain when you have some experience. Remember dair only spikes at the very beginning, look at the flash under Bowser as the move starts for an idea of where the lowest spiking point is. As the move starts above that point and goes down, that is the area in which the move will spike. Edit: forgot to mention one of the most important times to get a dunk. When you stand at the ledge and someone lets go of the ledge to use an aerial instead of performing a getup, a quick short hop dair read will spike all but a few characters for trying such shenanigans. If your opponent shows a habit of doing this, there's your read opportunity.
Don't be afraid to experiment, don't be afraid to go offstage and die. That's how you learn. Given an opponent who isn't great at edge-guarding, Bowser can often make it back from a good distance with only his up b. Know that at high percents the risk is greater. Perhaps at a certain point one might not want to risk going offstage. But then again, it can make all the difference. You can win the neutral onstage 2/3 of the time, but if the other guy wins offstage they'll probably win the match. Especially against certain characters with poor recoveries, good offstage game can be the difference between a comfortable 2 stock and a tense last hit situation. It's very valuable to work on. Most of this stuff is a matter of timing and pattern recognition. See it enough times and you'll know what a character's trajectory means, or what kind of recovery they're about to attempt.
There are generic recoveries and there are player-specific recovery habits. These all inform how you react. But that gets more complicated. While you're starting going offstage, don't worry about them air-dodging, just line up Bowser's attacks with where the other guy is. Air-dodging awareness can be added to your offstage bread and butter later. Even the top players don't try to wait for an air-dodge the first time they go into a bread and butter string of attacks. The other guy has to prove to the player first that they can air-dodge the attacks to begin with. Then the air-dodge 50/50's start. But in the meantime, keep it simple. You'll be surprised how many people get hit, regardless of their ability to air-dodge.
Edit: oh yeah, I didn't cover footstooling. It's not really a thing I do much, maybe someone else could speak to that (and anything else I missed). I should probably do it more.
Edit 2: if you haven't read it, you might want to read
@Big Sean's
Bowser guide. Though it's not up to date with the newest patch, it's a great guide.
Edit 3ish: so I have a youtube account I sometimes upload matches to (mostly me vs your average for glory players) and thought I'd post a couple of visual examples of what I'm talking about from matches. Some of these matches I'm not the most proud of necessarily, but they happened to be the first I found for the example of edge-guarding I was looking for. Since these are videos from full matches rather than a combo video, you can rewind a little for context if that helps.
-I don't have an example for a straight up runoff fair for the kill, I usually wouldn't consider that a flashy enough sort of kill to upload a replay of that.
-Example of runoff fair to bair setup, one that doesn't work
here, and later in the match
it works (though my opponent techs it). Each time I go offstage I'm making an educated guess regarding where Ryu will be. Though at this point I hadn't played many Ryus so I went deeper than needed the first time.
-Example of using a
bair to edge guard someone trying to recover high (which is to say, not dropping to the ledge), followed by a possibly unnecessary runoff fair. I don't have many matches with the straight up bread and butter version of this cause I try to only upload notable matches... Here's another
bair edge guard. Both my examples are a bit more flashy than I was talking about, but you get the idea. Ok,
here's one more. It's also possible to use a ledge jump to help with lining up a bair as I do
here. You can also hold the ledge and let go straight into a bair to catch people coming in low. Just be sure to get back to the ledge and onstage ASAP to avoid getting punished for regrabbing the ledge (and having no invincibility).
-Example of using
runoff bair to stage spike a vertically recovering opponent.
-Since it's in the same video, while I'm at it here's an example of
using dtilt to punish a poorly aimed recovery (this applies to any recovery that doesn't immediately sweetspot the ledge.
-Example of
dair dunk as opponent tries to get back to stage (with the air-dodge I spoke of). Bonus
flashy one cause why not.
-Example of an
offstage dair spike (don't do this though)
Ok that's enough for now.
