the basics explaining how it gives more power:http://auto.howstuffworks.com/question259.htm
on install: just follow the directions the that come with the kit.
And you must use the wet kits, NOT the dry.
On precautions (some of which included in the directions):
1. higher than 91 octane for 50 shot. personally I'd use 100 octane for any shot just to be safe.
2. 2 steps colder spark plugs.
3. WOT switch, an integral part of most kits.
4. Window switch, use the Mallory part # MAL-646.
5. Bottle heater/chiller: keep the pressure in the right range for safety/performance. some brands have only the bottle heater.
6. pressure gauge: again so you know you're bottle's at the right pressure for safety/performance.
7. for big shots like 75, I would retard the ignition timing, but you need some kind of computer to do this, piggyback (e.g. e-manage) or standalone (PowerFC). some people don't recommend over 50 for GTS, but others have done it safely, so it's up to you.
8. A/F ratio gauge with wideband o2 sensor/controller...not everyone can afford this, but just a good idea to make sure you're not running lean. you can test your nitrous on a dyno with a wideband o2 sensor.
but as long as you're not running lean already with a OBD-II lean code of p0171 you'll probably be ok, especially with the Zex which does a good job of getting the A/F mixture right.
Sorry I don't have time to explain everything now, but that's mostly everything I can think of off the top of my head.
btw, I installed a Zex wet kit on my friend's GT. it worked great with the 50.