I bought mine from http://www.dakotadigital.com/
Explore the website and I think you will find installation intructions. Any good RPM gauge you buy will most probably come with instructions.
The one I used just had 3 wires - a power source, a ground or earth wire, and a tachometer signal wire.
Hope that helps, if not, you will need to advise more details of where the gauge is needed, eg type of ignition used, stuff like that, John
Most after-market tachometers come with three or five wires. One will be a ground wire (black) one will be a power wire (red) and one will be a signal wire (some other color). There may be separate power wires for the lighting in the tachometer, you can join these with the power and ground wires. The tachometer signal wire is usually connected to either the negative terminal on the coil or the tachometer signal output if your machine has one. Later model Harleys have a good tachometer signal on pin 3 of the ECM, you can buy a pin connector from the Harley dealer and install it into the large connector that plugs into the ECM. Some Harleys will have a tach signal in the wiring to the speedometer, it is not connected and usually is doubled back inside the wiring harness. It will be a pink wire.
When wiring all this into your bike, it will be a lot cleaner if you wire a connector that you can separate if you want to (or have to) into the tachometer wiring.