Start building your robot by attaching the servos. You can bolt them on with the nut inside (left version below) or outside (right version below) the chassis. Typically, inside looks better, but outside is much easier. The choice is yours. Just ensure that the servo axles are both near the middle of the chassis.
Looks Better
(Nut on the inside of the chassis)
Easier to Build
(Nut on the outside of the chassis)
Next, add the caster to the bottom. Again, the bolts can be arranged to have the nut on the inside or outside.
Finally, use three standoffs (with a nut and bolt on each) to mount the Arduino Expansion Board to the robot. Once it's assembled, you can plug in the servos into the Arduino. Make sure they go on pins 10 and 11 with the orange wire connected to the blue pin, red wire to red pin and brown wire to black pin.
This is what the finished robot should look like.