A USB cable is not dissimilar to an audio cable, except that electrical power travels through the USB cable as well as data. To make a USB to Phono cable that meets the "specs" (specifications) for audio use, wire only the data connections on the USB cable. The procedure is straightforward and does not require any special tools, although some supplies from an electronics store or hobby shop must be acquired.
Cut off the connector at one end of a standard USB cable with the blade of a utility knife. Slit in three inches from the end on the outer insulation with the utility knife. Peel the outer insulation and the paper insulation beneath it back. Cut the outer insulation and paper insulation off the cable with the utility knife's blade. Separate the wires from inside the cable.
Heat up a soldering iron for five minutes. Place the RCA male plug on the work surface with the lug that is attached to the back facing right.
Strip a half inch from the ends of the white and green wires that are inside the cable with a wire strippers.
Solder the exposed end of the green wire to the lug on the back of the RCA male plug. Let the solder cool for a minute. Solder the exposed end of the white wire to the contact ring on the back of the RCA male plug. Let the solder cool for a minute.
Unplug the soldering iron. Wrap strips of electrical tape around the length of the wires so that they can no longer be seen.