Having trouble performing it consistently from a single jump, but I believe you have to release X a split second after you start to hold down. If you're familiar with other FG terms or notation, kinda like a kara input: X -> Down~release X
You can also practice with the slowmo options in training mode starting at 1/4th speed then trying to speed up your inputs when you move to the next highest speed if you feel like doing it normally is daunting. It's useful for confirming if you're doing it right since her dress moves differently while falling if you did it successfully. It looks like if you held the float and let go(which you are, but much faster)
I find that doing it a minimum height from a double jump is the easiest to get a rhythm for, since it's basically double tap X, hold down, release X which is how I'm practicing doing pseudo-wavedashes. If there's no technical advantage to JCFFing from a single jump(but that's probably a bit faster because you don't have to go through part of the double jump anim?) then I'd just try that and see if you can apply it easier.
Other than that I'd just grind it out. Start with just getting the JCFF, then once that's consistent move to JCFF aerial, JCFF at different heights, JCFF aerial at diff heights, etc. No need to try and apply it right away, just build the foundation.
If there's anything I seem to be missing/wrong about I'd love to hear it since I'm struggling a bit more than I'd like with this
