Staring with the battery holders fix all of the components on the frame using tape or hot glue. I put the Arduino on the batteries and the Motor Shield in the Arduino. With a screwdrived fix the motor's wires to the srew therminals of the Adafruit Motor Shield and solder a 5mm power jack to the batteries so you can power up the Arduino this way with 7.4 volts. In the next step you can see the circuit. It will be easier to explain that way.