Pretty straightforward. Just normal attack them to the edge, strafe to the edge before they are able to recover, and keep on spamming your normal attack. That will continue to keep them on the ground and unable to recover from the non stop attacks coming at them. Take advantage of this broken mechanic before it's too late. Here is the visual representation. Credits to the original creator.

Sandy cheeks isn't hard to learn and is by far one of top characters to easily learn in my opinion. Playing on a keyboard took me a bit to get used to, but not that long. Her DownSpecial, Lasso, and NeutralAirSmash, in my opinion, will be the foundation of her playstyle and combos can pulled off from that. Anyways, here's an image with my keyboard layout:

A: Attack
S: Strong attack
D: Special attack
[Space]: Jump
C: Defend
V: Grab
[RCtrl]: Strafe (Broken). I normally do c+[spacebar] and choose the direction I'd like to go. This method works best for me
T: Taunt

FYI, you can check out the combos in the extra section in game. Just to get the basic understanding as to what she does. Any combos that I come up with, I'll make sure to update this guide. But honestly, just keep playing and analyze yourself.