If you have a voltmeter (not an ammeter) you shouldn't require a proper wiring diagram for hooking up the gauge.
If it's a Smiths gauge, either gauge terminal can be connected to chassis ground (preferably with a black wire), the other terminal connected via a green wire to any green wire you find behind the dash. If the gauge is a brand other than Smiths, the GND gauge terminal (perhaps a black wire) goes to chassis ground. As before, the remaining gauge terminal gets connected to any existing green wire behind the dash. Fuses are optional. If you want one, put it inline with the wire going to the car's green wire. Use something small, like 3 Amps.
Illumination can be one of two colors, red or red/white. Look for existing illumination bulbs behind the dash and splice your voltmeter's lamp wire into one of those wires.
Green wires are switched, fused wires in Lucas schemes. Connecting your voltmeter to a green wire means it will only be powered up when the ignition key is in the run position.
If you really meant ammeter, that wiring is totally different.