It's just L or R and then jump. You have to jump within 5 frames of pressing the shield button, and you have to press L or R fully (the button should click) in one frame. If you take longer than one frame, you will not get the proper parry. You should trigger trick if you don't, since this makes it easy to get the 1-frame full press of L/R. Just hold down your parrying button fully when plugging in your controller.
As far as using it in real matches, consider some of the following:
- first, make sure you can jump out of parry. Turn off the music and go into training mode. Press shield, then jump. If you hear the shield sound and you got the jump, you're doing it right.
- get the tech skill down 100% first. Go into training mode and parry red shells, or practise against level 9 Ganon, or have someone shoot Falco lasers at you. First practise just jumping out of the parry, then practise follow-ups. The all-purpose one is DJC Nair, so start with that and learn others later.
- think of the parry and its follow-up as one technique. You should be prepared to attack before you start the parry, because you don't have time to confirm that you got the parry.
- once you have the tech skill, find a player who also has tech skill but does the same things over and over. The easiest character to practise against is Falco because noob Falcos will spam lasers and early Dairs. Practise against someone who does this until you are comfortable with parrying obvious attacks. Be careful not to get into the habit of always crouch parrying, because good players will wreck you for that.