There's only one z-cancel, being the one where you land.
Basically, this combo is not a magical win-button that you can push... you have to be really good at the basic technical stuff and quick with the fingers before you can really use it. The only factor is speed. Practise it in training mode on 1/2 speed, then move up to 3/4, then finally full speed... but expect to take like... a couple days doing it on 1/2 or 3/4. If you're not getting it, then just try to improve your game in general: use more short-hops, practise combos, blah de blah. Leave the combo alone for a while. Then, as your fingers get better, try it again.
A good way to practise z-cancelling right before hitting the ground is to go into training mode against a short character like Kirby, and practise short-hopping and uairing him right before you hit the ground. Don't even bother with the drill, just practise pressing A+Z (or R+R in my case) right before you hit the ground. Whether or not the Kirby gets hit isn't terribly important; you just want to make sure Mario doesn't do his little recovery animation when he lands.
