The tach usually comes with instructions for this. I'll try to recall this as best I can, and I won't be able to help you on which wire color goes where, I don't think.
The Red wire goes to a hot wire, a wire that carries current whenever the car is ON.
The green (i think) wire goes to the positive side of the coil?
Black wire to ground.
There should be one more wire that goes somewhere into the headlight circuit to turn on the little internal light for nighttime stints. This is where you need the specific wiring diagram for the car; I'd scan in the schematic for my '67, but I'm not sure it's all the same...
It might be wise to invest in a shop manual, though, because not only will it have all the wiring diagrams for your car, but also pretty much everything else for working on your Stang.
Sorry I wasn't more help, that's all I can think of. I'll go out and take a look tomorrow