What you need:
(2) 12v batteries
(2) DC 12V motors
A speed control: Sabertooth 2x25
A receiver and a transmitter
An extra battery for the receiver (You DO NOT need this if your receiver use less than 5 volts)
some wires
First, connect the motors to the speed control (sabertooth 2x25)
Next, connect the speed control to the receiver.
And then, Connect the (2) 12V batteries to the speed control as you see above.
Last, connect the 6V battery to the receiver.
You should be ready to go!!!
Shoot me questions if you need to.