Hi David,
Tom ships the voltmeter mounted on a 'tin' that is the same as what is in your gauge cluster already (assuming you have a cluster with gauges rather than idiot lights). He also includes the wires for getting power from the temp gauge that mounts next to the voltmeter on the same tin. So, you move your temp gauge to the new tin provided by Tom, and wire it up. See
post #1 here for more info.
Hope this helps,
Jon