Here is the principal and most important thing about the beetlerobot.
It is call the "third connection"
In you AA/AAA battery you have 2 wires that provide 3Volts.
In reality, we only use 1,5Volts for both motors.
How can we get half the voltage? Simple, you solder a wire like the picture below. The new wire will touch the last wire you have solder in the last step and you will connect it to the battery holder. See picture.
Be very carefull when soldering to the battery holder, you can melt the battery holder! I have already done that. This will happen with cheap AA holder.