Heyyo, so I guess I'll help you out with this as best I can seeing as how you haven't gotten much attention. Alright so first off I'm going to direct your attention to the name of the tech you're trying to learn, it's called Rising storm and the inputs are quite simple
full hop>instantly needle, if done properly you'll have about 10-20 frames to act before you touch the ground I'll link a gif to it at the end of the post, as well as a thread with a little more info on the tech.
Now to practice this I suggest going into training against bowser (easiest target to hit). Start by simply standing about a dash away and doing it a few times, once you've gotten it down standing still practice it off running towards him and finally set his damage to oh 60-80% launching him with say sweetspot nair and doing it offstage (better yet get someone to help you with this last method) if you can consistently do the last two methods you'll be ready to do it in matches. Finally the connection is quite simple but you have to realize it's only guaranteed if you compensate for their DI, luckily there's not a lot of things they can do to try and escape this and it is a true combo so as long as you read their DI and their KB you'll land it every time at almost any percent. Just remember practice makes perfect oh and doing this all the time in casually gameplay is amazing as it's just oh so safe.
For future reference can you please direct simple questions like these to the general/new player stickied threads? I know Tristan would like that.
Best of luck!
GIF of Rising Storm (Credit to tristan our lovely Moderator for discovery and Waisser for the amazing gif):
http://abload.de/img/risingstorm3iktd.gif
Thread with list of AT and more:
http://smashboards.com/threads/the-...ed-techniques-combos-and-tricks-wii-u.371988/
Thread with lots of cool links for us sheiks:
http://smashboards.com/threads/sheikah-catalog-time-to-step-up-your-game.387205/