This really isn't a big deal. Any generic voltmeter with a suitable range will do the job. Drill a suitable hole and mount it in the dash. The one in the photo came from a direct-from-China seller on Ebay. This is a similar example:

https://www.ebay.com/itm/LED-Digital-Display-Voltmeter-Car-Motorcycle-Voltage-Volt-Gauge-Panel-Meter-12V/382624102408?hash=item59162c7808:m:mjOteNyJ32wi3AcLHzv4Mzw

A voltmeter has two wires: one goes to any ground, the other goes to any switched 12V source, I think I used the lighter wire for this installation. If you want to be anal, you could run wires all the way to the battery, but it's not really necessary.

As far as reading it. A fully charged battery reads 12.5V. Above 13V means the alternator is charging. Bosch alternators are usually regulated to 14.25V @70F. They will vary from 13.5V to 15V depending on operating temperature. If the reading drops below 12.5V, it means the battery is discharging. There are extreme conditions when this may be ok: idling for a time with the headlights, wiper and heater on, for example. So much more information that you get from an idiot light.