Those links are good as a guide.

What you really need essentially is a place to hold the water, a pump to push it, a nozzle or 2 to distribute in a mist, and a switch to contol it.

Tank - I have a coolant return tank mounted in my trunk
Pump - I have a windshield washer pump mounted directly on the tank with self tapping screws and silicone to seal.
Switch - I have a cheap momentary switch on my dash to turn on the pump as needed.
Lines - cheap plastic line I think it's 3/8" ID (look for what fits the pump and get enough to reach)
wires - to ground the pump and run the 12+

You can make any trigger your "switch" it's up to you how eleborate you want to get. For instance mine only works with the ignition on since it gets 12+ from the ignition.

It's really up to you how you want the system to function, it's just a basic water pump. I actually rarely use it and will probably install watergun nozzles aimed outward to spray people as I drive by.

I was going to use these same concepts to build my own meth/water injection kit (basically the same concept, to pump a fluid from a tank) but I picked up a sweet Aquamist sponsorship - more details to come...