There will be three terminals on the back if the tach. One will be marked power or ignition. You run a red wire from this terminal to the "I" or "A" terminal on the ignition switch. This supplies power only when the switch is in run position.
Ground is attached to any good ground under the dash. Now, Color might vary a bit depending upon ignition but it is usually the grey or purple wire in the cable from the engine which is attached to send or signal. If you take the cover off the engine there should be a wiring decal on the exhaust cover.
