Hey there, again!
The in-air Force Palm stuff can work against any character at any percent either on its own or after any move/canceled into from an aerial. You simply have to be close enough to the character so that when the command grab of the side-b comes out, it grabs them in the air. Pretty much any moment where you can press side-b in the air in time to grab them it will work, tilts and aerials both.
Now the setups I've used to create it are fair, dair, or a read into which I just jump/fall into the opponent and side-b. Most of the time when setting up a force palm, I position myself directly above the opponent and fair and cancel that immediately into a side-b. I have found that this is guaranteed as low as 40% (even lower with heavies) and can go up to around 80%. At that point I'd experiment with dair. I rarely use dair, but I'd like to more often. If you watch IPK, this is the setup he goes to most often, I'd say you should study him to get a better grasp on that, as I don't use dair enough to truly know its exact workings. Fair is my go-to.
Basically, if you are looking to setup a side-b rather than a nair, you want to combo your opponent so that once they are offstage, you have enough time to get above them and pull off that fair cancel to side-b. There's no exact set-up, just anything that leads to these final two-moves. I have had my fair share of 0-deaths simply by approaching an opponent near the edge and doing dash attack canceled into grounded side-b grab, jump to fair above the opponent, then cancel the fair into another side-b. The final side-b should be charged into a spike.
Remember, side-b uncharged is a meteor, which is viable to kill at high percent or against opponents with bad recovers, while charged (press a after the aerial grab), will spike and guarantee. UNLESS, you are playing a good Luigi, a good Luigi can actually recover from a side-b spike if it is done while Luigi is under 40% (I know from experience, yes, this actually happened to me).
Finally, side-b is also something you just have to feel out. Sometimes I get raw side-b spike simply because a read an opponents movement offstage. Sometimes I read their jump so I jump and side-b, and they jump right into it. As you use it more, these will become instinctual. Other times, you'll be able to successfully set-up into it, and you'll just instinctual do fair-->side-b or dair-->side-b. Just keep practicing it, and the better with it you'll get.
Edit: Also, if you ever drop by a Project M Club at UC Davis event again, find me and I'll be glad to show you what I know. Just ask whoever is hosting the event if they have seen Ghost, they'll know who you are talking about.