12v motor wiring

I am running trying to run a 12v stepper from a Diemicila. A 9v battery seems to be running the sketch but I would like to learn how to add a higher remote voltage. Using the two wire schematic for a bipolar at http://www.tigoe.net/pcomp/code/category/arduinowiring/51, the external + power supply connects to V2. Where do I connect the ground wire from this source? Thanks.

Sorry, I forgot to mention that I am using an L293D H-Bridge.

Ground should be connected to one of the 4 ground pins on the L293.

You also need to connect Arduinos ground and the L293's grounds together.