Chassis- Depending on how tough you want your robot to be, choose the material for chassis. You can use a piece of wood or metal or bakelite sheet.
Motors: Depending on the speed you want select the motor’s RPM. A 100 RPM DC geared motor will be good enough for general purpose. In general for DC gear head motors lower the RPM higher is the torque.
Attach the clamps to the bottom of the chassis, where you will connect the motors and the wheels.
Also connect two motors on same side in parallel. Double check that the two motors connected in parallel rotates in same direction when potential difference is applied on there connected points.
Fix the tyres.
Then connect the wires to the dpdt switches and the smps.
Here the dpdt switches are mainly used for changing the polarity in the motor, so that we can move the motor forward or backward with ease.
Your bot is ready. Drive it!!