Most MP3 players, including the iPod, use the Tip for the left channel. Solder the Blue wire to the tip connector, the Red wire to the Ring connector, and both Gold wires to the Sleeve connector.
When all wires are soldered to their respective connectors, squeeze the cable tab at the sleeve connector tight around the cable. This helps to keep the solder connection when the headphones get disconnected by pulling on the wire.
Attach the cover to the audio jack connector.