Advanced Autonomous Kit for Sphero RVR Assembly Guide
Mounting the Servo Horn
While the pan-tilt will be facing up, the servo horn (which the servo will be affixed to) is mounted on the bottom of the plate.
Line up the small holes on the plate with the small holes in the servo horn the same. Use the small, self-tapping screws to mount it in place from the top of the mounting plate.
Two of the small self-tapping screws will suffice, but if desired you can use all four.