Practically there is nothing special about measuring voltages with Arduino. Simple analogread() command already takes care of getting values and converting them to voltage using coefficient. Anyway for some starters this might be quite educational to build something similar.
This voltage meter uses four Arduino analog inputs. Each channel is equipped with resistor voltage dividers that scale down voltage 10 times. So practically 5V powered Arduino can be used to measure voltages up to 50V. Voltage values are displayed on standard LCD which already has library for fast use. If you need to tune the measured values there are calibration constants available in sketch where you can individually select to make Arduino voltmeter pretty accurate.