StickyChopstix: It depends on the aerial and where you're trying to hit with it. For shffl'd fairs, I short hop, fast fall, then tap to the side with the C-stick as I'm falling, then L-cancel. For shffl'd (and auto-canceled) nairs, I short hop, tap A, then fast fall. Also, for dairs out of shield, I attack before I fastfall, so I can do it while rising, letting the attack come out as fast as possible.
kupo15: Frame data's not that important, honestly, until you get into more complicated things. If you want to learn the timing for the shdf, go into training mode and use 1/4 speed. Note where your fairs are coming out, and emulate that, only at normal speed. The most common mistake is to try to do it too fast, or through button mashing. Find the rhythm and use it.
Auto-canceled nairs are cool. You have to land after the second slash is one though. I L-cancel it anyway, just in case I messed up the timing, and anyway, it's not that much more effort.
Marth has some weird hitboxes sometimes and you get weird tippers, like utilts from behind. You just have to get the feel.
Keep in mind, though, that the "boom" tipper sound is only for the usmash, dair and fsmash. His other attacks have different softspot and sweetspot sounds.
Oh, if you really want to see where the sweetspot and whatever else spacing is, find Seanson's hitbox system. It's pretty good.