Grab the right-angled stereo plug to pigtail cable and measure up the length you'll need. If you cut it too short, you can always extend by splicing with a longer wire.
You'll want to remove about 10mm of insultation from the audio cable. Use wire cutters or a hobby knife to strip the wire. Carefuly bundle the stranded wire and twist it together to make a uniform wire - this is the ground connection.