I believe this is probably easier to do on a pro controller, but it's totally doable on a Gamecube controller. I'm still bad at doing it on stage (I don't practice like I should), but I get it almost 100% of the time that I want it off of ledge.
Are you having trouble with both variants? Doing it from ledge is waaay easier because of how the Z input buffers after your jump from ledge input. I can't tell you the exact frame window for putting in the zair input from ledge, but it feels like you have forever to do it compared to just a grounded one. Practice the ledge one before you do the neutral/grounded version for two reasons: it's much more useful from ledge because it gives Lucas another get up option, one that confirms into a grab at low percents, and because nailing the timing for it helps you understand the inputs on ground. Just remember that if you do it from ledge, you lose your double jump so if your opponent reacts and knocks you off, you're going to have to PK Thunder or airdodge back.
The grounded one is harder mostly because of how soon you need to input the double jump after your first jump, which also has to be a specific height to get the true effect. Keep in mind that to properly perform this tech, you have to be holding a direction on the control stick - any direction. I practiced mine by just holding down so that I wouldn't go anywhere if I did screw it up - I'm not sure if that was just good for my mind or if it really did help me do it better. When I started getting the DJC zair about 5 times in a row consistently, I started holding forward or backward with it, which honestly feels like it messed up my timing. Once you've done it once, though, it's all about remembering what that input felt like and getting it to be second nature.
