There are multiple grounding points, but for some reason some of those didnt work if used on this. For example left joystick legs did not work for grounding, but legs of the right one did work. Just start with right joystick legs. Thats what is written on the tutorial and it worked for everyone that did this mod.
And you dont really "ground" them. You activate button by connecting it to ground. Those 4 buttons are active low like almost all buttons.