The air fuel guage should have come with instructions.
The red wire is 12v signal
The black wire is a ground
The purple wire is your o2 source

If you have an obd2 vehicle I recommend you tap right into the white wire of your 02 sensor that runs into your exhaust manifold.

To be honest with you air fuel guages wont tell you shit. Why? Because your stock o2 sensor only reads a limited amount of air fuel ratio.

You need an expensive tool called a wide band oxygen sensor in order to get real air fuel mixture readings.

Good Luck.