Tach wiring
Where & how you mount the thing is up to you (personally I prefer gauges visible to the driver while inconpsicuous to anybody else), but you ought to get a decent enough tach that has clear instructions. You can also download generic instructions off the web; just be careful in applying those.
But as far as your signal lead, you're better off connecting it to the tachometer output from the ECU--as opposed to running a wire all the way into the engine compartment for a hot source like your coil or ignition; you want to avoid that if you can.And if you get a good tach, you can.
Simpler & less risky is always better.
Even cars without tachometers have a tach output at the ECU. You can locate this lead in a Haynes manual (acceptable, for around 12 bucks) or the specific Nissan shop manual (much better, for 50 bucks). Alternatively, you might be able to talk a sympathetic parts guy at a Nissan counter into telling you which output to use. But you need to be careful connecting anything to your ECU; that is your engine's brain. Nothing fast or clumsy, no jumpers that might work loose, nothing that might ground out. Easy does it.