The datasheet for the voltmeter should tell you what resistors you need for it (and how to hook them up) based on what your intended voltage measurement range is. You probably want to use the 19.99 volts range since it's got a 3 1/2 digit display (ie, 1999).
As far as where to connect it...for best accuracy you might want to run a wire directly to the battery positive terminal. If you don't care about accuracy, then just connect it to the clock or radio power wire.
