I built this small “Robot” a while ago.
It is built on the Tamiya base plate, tank treads kit and dual gearbox. Uses four AA batteries and has an Arduino for brains. The motor controller is a L298 dual H bridge.
The schematics are extremely simple. The L298 is connected directly between the motors and the Arduino, and only requires an additional pair of resistors (10Ω) to work.
The Arduino Sketch allows it to travel Forwards, Backwards and turn at two different speeds. PWM could be a further improvement.