hemi wireharness cut/splice ?

Ok, i'll admit im a little dodo bird here with this wiring and the new hemi.
I keep hearing everyone say you gotta splice, and cut and make a harness, etc etc.

What exactly has to be done to the harness? In my mind all I see is needing to splice the wires into your existing car harness..ie brake lights, headlights, radio, igintion..that kinda stuff (some might not even be related)

Am I missing something? or are you having to, for a lack of a better word, having to "hack" the wiring to make it run?

Ok now if that is the case, I don't see it as beeing so hard.
now my personal situation, I am no stranger to wires. i do IT work, don't make me an electrical engineer by any means, but im not scared to splice adn solder and whatever.

Also, my car as is, is going to need to be completely rewired anyway, as the guy who had it before me hacked the hell outta the wires...i don't even have an engine harness right now.

If anyone can clear this up a bit, I would appreciate know what EXACTLY is needing to be done.

Thanks