Voltmeters ideally should have infinite resistance . Ammeters ideally should have no resistance.
If a voltmeter is not ideal than using it in parallel with a component would cause the voltage measure to be less than what it should be as current is drawn from the main circuit. The measure resistance of the component using a non ideal voltmeter would give a value lower that the actual value.
If a ammeter is not ideal than using it it in series with a component would increase the resistance to the measured as the total current would be less than the actual value.
