In this video, the author is demonstrating how to build a simple Arduino on a breadboard, based on Atmel Atmega8/168/328 microcontroller.
When connecting your Arduino to a power supply or a battery make sure that you connect the red jumper wire to the positive terminal and the blue one to the GND. There is no protection against reversed polarity.
Circuit Diagram: click here
Do you want to know how to program this Arduino: click here