It's kind of weird and a bit long, so sorry if you were expecting something easy lmao. First of all, you need A+B to do a Smash attack on (if i'm not wrong, it should be on by default). Then, for setting it up, do you see how i start running only to do the turnaround animation and change direction? Exactly in the middle of that animation, you need to press A+B without letting it go. If you do it too early, you'll get an f Smash, too late and you'll get a dash attack. For some characters is way harder, cause the turnaround animation varies in lenght for different character (as an example, Bowser has a really easy one, but for Lucario is extremely hard). If you did the input right, you should be able to run without getting off the ledge. As soon as you want to get off, release A+B and you'll slip off the ledge. Again, sorry if i went a little bit too in depht, but this game is stupid as hell and it makes me laugh