The move itself is very very simple. It stands for Dash Attack Cancelled into Up Smash. It's pretty clear how to do it from that.
This is what you do. It helps to think of it as a 1, 2, 3, 4 motion.
1. Forward
2. C-stick Down
3. A
4. Up.
While your left thumb rotates the joy stick from Right/Left to Up, your right hand hit's C-stick Down and A. Tap Jump OFF makes it easier, otherwise you'll find yourself jumping and d-airing a lot. If you can time it right, though, and hit A a fraction of a second before Up, you don't really need to.
As soon as Sheik begins the Dash Attack, input the Up Smash. You have BARELY half a second to do it. Timing is everything here (And for a mommeennttt the worrllddd disappears. Sorry, song I like)
Anyway, practice practice practice. You have to pretty much hit Cstick Down and A at the same time, and A and Up at the same time, but at the same time, it has to be 3 different inputs.
NOTE: Feel free to change the button config. I always forget to choose my player, so I didn't bother learning it that way. A popular config is set Z to attack. To make up for the grab issue you can make L or R grab.
Assuming Z is now attack.
Forward > Z > Up&A
Basically, it's just this.
1. Forward
2. Whatever makes you dash attack.
3. Whatever makes you attack. 4. Up.