1. Slice open the insulation length-wise near the male A connector, the one that attaches to your computer.
2. Pull out the bundle of 4 wires and two shields(braid and foil).
3. Dig thru to find the heavier red and black 5v wires (the thinner green and white wires are for data).
4. Snip off the red and black wires, sealing them shrink-tubing or tape. Otherwise, you risk shooting 5v thru the ground plane. It will mess up your USB bus.
5. It's a good idea to roll some household foil over the bundle to ensure continous shielding. Tape it all up and hook it back up. You should hear a reduction in the "scrim" that seems to ride on top of computer audio.
6. Some WARNINGS: Try this fix only with DACs that are AC powered. They usually don't need the 5v. Also, try this first with an expendable USB cable. Of course, after your DAC turns digits into analogue, hook it up to a Roy Mottram tube pre-amp and/or a Bob Latino tube power-amp!
Note: I discovered this reading a TAS review of a $900 Audience USB cable. They separate the power and data wires with completely separate shields, jackets, and male A connectors. My way is cheaper.