The 'voltmeter' we normally use is really a very sensitive ammeter with a large series resistance.
What the 'voltmeter' is doing is measuring the small current that flows through it (a serial measurement) and converting this current value back into a 'voltage' reading (i.e. the voltage across the terminals = voltage across the 'voltmeter').
By Ohm's law (V=IR) we know that V is directly proportional to I, so its just a matter of how big R (the constant of proportionality) is to convert one to the other.
It's much easier to measure a current directly (even very small ones) than it is to measure voltage directly.
Examples of 'voltmeters' that are really ammeters:
A moving coil multimeter: A low cost moving coil 'voltmeter' is typically a 50uA movement ( 1000 ohm coil). For a ONE VOLT reading (full scale) we need to have a total resistance of 20K. (you'll sometimes see this marked on the meter as 20K/volt) i.e 19k (external) + 1k (meter).
The range selector switch adds suitable parallel/series resistance into the basic meter circuit
A digital multimeter - requires/measures a much lower current something like 1M0 per volt and is closer to an 'ideal' voltmeter which would take no current from the circuit (infinite resistance?).
The resistance of the 'voltmeter' is only really important if the current taken from the circuit significantly interferes with the reading. In the case of a battery (very low internal resistance) this is highly unlikely.