A voltmeter is simply connected in parallel with your battery. Simply run a cable from the +ve on your battery to your voltmeter and and one from the -ve on the battery to the remaining connection on the voltmeter. If when you start the engine the meter reads a negative value just reverse the connectons.
NO NO NO! Take the negative of the meter to earth, and take the positive to a switched ignition point. This is easily found with a meter. This will ensure there is no chance of a fault, or the meter flattening the battery overnight.