Adding Free Item to Orders Over Certain Amount

I have question concerning this. There have been a few threads in the past made about this, but nothing too helpful.

Apparently, this is the tutorial and way to implement this.

http://www.zen-cart.com/wiki/index.php/Developers_API_Tutorials#A_Real_World_Example

However, my question is, where does one implement or place this code? A step by step guide would be helpful, but even just knowing which file to place this code into would be good.

Also, in the tutorial it says, "With the ONS, this can be achieved with one very small custom class and absolutely no hacking whatsoever." What exactly is this ONS? I'm sure it's not a one night stand.

Thanks

It's the Observer/Notifier System. Using it will require knowledge of modern object oriented coding practices, very simple for those who know how, gibberish for those who don't.