The finished circuit can display numbers ranging from 0 through 1023, in connection with a 10K preset pot attached to the analog input A0 of the Arduino board. This 4 -digit 7-segment display section ...

Just follow the schematic circuit diagram ... is added between ground rail (0V) and the common cathode pins (3 & 8). It is better to directly connect pins 3 & 8 of the display to ground rail. Next add ...