C Wire to a Dual Zone House
If you have a spare wire at the stat then half the battle is done. Now you need to confirm it's viability at the source be it furnace, boiler, air handler etc.

If the connected equipment is a furnace or air handler and is under 10 or so years old it will almost certainly have a common terminal on a board. Not all boilers have a convenient C terminal. 

You simply power down the system and connect the wire in question to the C terminal. Make sure the wire at the stat end isn't contacting anything. Close the access door, power on and at the stat test from R to the wire. If you are reading around 23 - 28 or so you're good to go. Power down make the necessary connections to the Nest and power up.

If your situation differs from the scenario outlined above, edit the question with appropriate details. Good luck!