Go to battlefield. Fullhop and buffer dair, if you land with lag, you did it too late. The platform is the max window to get the auto cancel. Practice doing full hops and starting the dair at the climax of your jump. Try and get it as late as you can. Once you have a good understanding of the move, forget about trying to get the max range, instead focus on actually hitting your opponent with it. When you use it in neutral, try and make it so the last hit of dair hits their hurtbox. If you focus too hard on getting the max range, you won't be focusing enough on actually hitting them, or using it at the right time.
I think this logic applies with almost every other move you use in neutral. It's more important to focus on your opponent than yourself, otherwise you'll go on auto pilot.
Edit: I think this is why players like mango are better than super technical players like westballz. You can multishine a shield all you want, but that doesn't really matter if you can't outplay your opponent.