If you don't have it already, I'd highly suggest figuring out how to install 20xx, since it's entirely designed around solo practice. It's much easier to learn and know when you're doing stuff right/wrong with the tools in it.
Spacing Aerials
Start by practicing on stationary opponents. Learn the ranges for your bair/fair and aim to hit the opponent with the tips of them for the best spacing. Then practice weaving in and out with those attacks, still keeping sure to keep your opponent at the tip of your range. This is Puff's biggest advantage in a majority (if not all) matchups in the game, so this is the most important thing to master.
L-Canceling
20xx has a super handy feature where you can tell it to flash white on successful l-cancels, and flash red on unsuccessful ones. If you don't l-cancel already, it'll feel pretty unnatural when you first start adding it to your play. Start by learning to SHFFL (short hop fast fall l-cancel) aerials to learn the timing for l-canceling. Basically just short hop, throw out a move, hold down, and hit L just before you hit the ground. Once you get the hang of that, practice it on a stationary opponent. Due to hitlag from hitting a character/shield, the timing will be slightly delayed from a regular SHFFL, so make sure to know the timings for both. After that, practice throwing in up-tilts (or any other move/action) right after the l-cancel, first on your own and then again with a stationary opponent. It takes a lot of practice to get used to at first, but it slowly start to become second nature once you get the hang of it.
Autocanceling
To be entirely honest, this is one of the things I know least about so if someone else wants to step in with more frame data knowledge than me/corrections to what I say, feel free. I'm pretty sure Puff's nair autocancels at like frame 29, so landing about half of a second after using it will make it autocancel. Take a look at the Puff frame data thread and it should have the timings for all of Puff's autocancels. With 20xx's l-cancel feature turned on, if you do it correctly then Puff will just land with no white/red flash at all. Really it's just a matter of memorizing the timing of the move's animation and knowing when to land.
Dair->Rest/Grab and Rollout to Ledge
Rollout to ledge is kind of more of a gimmick than a really helpful recovery option, I wouldn't really suggest using it at all. Yeah it looks cool, but if you mess it up then you kill yourself and if your opponent expects it they can just jump out and get hit by it and you're left helplessly falling to your death, a super easy edgeguard for a generally un-edeguardable character. Here's a video on it anyway though if you're still interested.
https://www.youtube.com/watch?v=gOxry4EY9Gg
As for dair->rest, this one is much more useful but still a little hard to pull off. You're aiming for the last hit of dair to hit the opponent's bottom-most hurtbox, which is like under their feet. Hard to hit consistently, but still doable with practice. It's possible to do on every character, but how easy/hard it is depends on each one. Personally I find the most success on Fox/Marth, but it may be different for you. Going for a dair->grab is the much safer option (Hbox only does this, I've never seen him do dair->rest), since you pretty much will never miss the grab. Here's a video on dair->rest that explains the timings and percentages that it works on (it's the same for dair->grab too).
https://www.youtube.com/watch?v=57ynH7mqqF4
P.S. I'd suggest watc