The 3.5mm stereo audio plug is the standard jack for MP3 players and other portable audio devices. This mini-plug was developed because a lot of modern equipment cannot accommodate standard 1/4-inch jacks. The small size of the 3.5mm plug makes wiring one a bit difficult, but with practice and the right soldering equipment, most hobbyists can do it.
Slide the shell of the mini-plug onto the cable. Do this before you start stripping the wires; this way, you only have one smooth cable to thread through the small shell instead of four wires.
Using a sharp knife, strip off about an inch of the cable's sheath. Then strip the tubing surrounding the inner cores.
Twist the two ground wires together. These are the bare wires surrounding the two inner cores.
Tin the cable cores and the ground wire by touching the solder to each one and then applying heat from the soldering iron until the solder flows onto the wire.
Put a spot of solder onto each terminal of the mini-plug.
Connect the wires as follows: left channel of the mini-plug to the tip connector (centre connection), right channel to the ring connector (middle terminal) and ground wire to the remaining terminal.
Connect the plug to the cable by touching the tinned wires to the terminals and applying heat. The solder on each the tips will run together forming the connection.
Slide the shell over the terminals and tighten it.
You'll need a narrow-tipped soldering iron because both the terminals and the wires are small.
Make sure the solder on the terminals does not touch---the plug will not work.