The voltmeter will need an ignition switched live feed, earth and feed for the backlight

Oil temp, my nomad comes with a replacement sum plug with a threaded hole in the centre that the temp sensor screws into, a wire is then taken from this to the gauge, also an ignition switched live and lights feeds will be needed.

Oil pressure, should come with a fitting for the sensor that T pieces in where your existing sensor for the low oil pressure dash warning light is, then needs ignition live and lights feeds.

Cheers
Chris