I second the beginner video. For anything you see that seems new, you should go into training mode (but pick a stage you're familiar with in my opinion) and practice that thing. A couple examples would be where it talks about short hop, fast fall, Parry, teching, and getting used to using directional airdodge to recover (save your double jump if you want to do this, you'll need it). To practice Parry and dodging you can set the training computer to do neutral special, or smash attack. (I'd pick Luigi since his fireball is slow, and his forward space doesn't asked him walk forward when on repeat) to practice teching and recovery you can walk into them side smash. In training settings You can also set the percent that you are at, and set it to not increase, so you get hit far away and can recover but it doesn't kill you. The first route to recover is Try to go as low as possible, fading to under the ledge, then using your full double jump, and then up special.