You don't have to
Voice Command is speaker independent. It's smarter than the dumb tag dialers.
You can just say "show eric badger" to show the contact called "Eric Badger"
or "Call Eric Badger".
You have to first assign a button. Most people use the button on the side of the device. Use the button control panel applet to associate the side button to "Voice Command". Press and hold the button until you see the microphone icon pop up and an audio cue. Then say "Call <contact name>".
You have to say the first and last name exactly as it is listed in contacts for it to work properly.