It's pretty easy, and you have 2 options
1) This is the easiest...just get a 1/8 inch male jack to dual female rca converter from radio shack. Then, simply run normal RCA cables from this adapter (which plugs into your sound card out on the computer) to any auxiliary input on your receiver. Then, simply select that input on the front of the receiver, and you're good to go
2) If your soundcard has a digital (SPDIF) out..you can convert it and run it into a coaxial digital input on your receiver (assuming your receiver has digital inputs). If you do it this way, and enable digital out on the sound card, you will get better quality than using method #1 above. If you're sure you have both a digital out on your soundcard and a coaxial digital in on your receiver, then let me know and i can let you know exactly how to hook that up...else just stick with method #1