Add ball caster
Add a ball caster to the bottom of one of the short ends of a mounting board.
Add decorations if you like. I added the circle attached to a long rectangle from the Gadgets and Gizmos kit.
Starting from the end with the ball caster, add battery, power, Bluetooth bit, and the splitter.
Connect one motor directly to one output from the splitter. Connect a wheel to the motor and attach it to the mounting board.
Connect the inverter to the other output from the splitter, then attach the other motor to the inverter. Attach the other wheel to the motor and attach the motor to the mounting board.
Set the motor on the right to CW (clockwise). Set the motor on the left to CCW (counter-clockwise).
Pair the bluetooth module with a button in the littleBits app. The button will switch between the left and right motors. The robot will move in a circle, but if you toggle the button on and off, you can get it to move forward.