I assume you are going to mount it in/on the dash.
Find any wire under there with 12v on it and find a ground wire. Hook the meter across those wires. If the meter is not marked as to which terminal is + you might connect it up backwards which will cause the needle to peg off scale.
The current required to run the meter is almost immeasurable so any small insulated wire will work.
You need only two connections. +12V that is on only when the key is on, and ground. The +12V connection on the gas gauge or tachometer would be two points to pick up switched 12V. The gauge measures battery voltage so there is no direct connection to the engine..
