Well, once you've confirmed that whatever you're running has init.d support, it's just a matter of creating a blank file somewhere (name it 00awhatever as init.d executes files in alphabetical order), filling the file with the code given, copying it over to your init.d folder and setting proper permissions.
The script should run the next time you boot.
Keep in mind if you have an init.d folder filled with stuff, it's gonna be a bit more challanging as you're gonna need to figure out load order (what needs to run before what) if something goes wrong.
And if any of the above sound like alienspeak to you... then it's probably easier (and safer) just to flash the given zip... as long as your device supports being fed boot scripts as per the directory structure that the zip is built on.
And yeah, I have meefix's Busybox installed in /system/xbin/, if that helps.