proper soundcard to amplifier cables

hi all

i've always had a bunch of makeshift cables with 1/8 jacks on one end and dual rca jacks on the other end, allowing me to pipe my sound card to my amp, but i recently noticed my polarity was crossed. i did a quick hackjob and just flipped the connections of one of the speakers on the amplifier, setting things straight.

i know how to make it work, but I want to do it "by the book". can someone please tell me what exactly to connect to what to give me a "proper" 1/8 -> dual rca jack?

also, the cable i'm using is STP cat 5. should i run ground through the shielding as an unbalanced load, or should I just have one wire on every pair be a ground. i would check it out myself to see which'd give me better sound, but i dont have an oscilliscope.