Your Headphone jack should have 2 pins, and a bigger one at the base
BEFORE YOU DO ANYTHING slide in the jack cover, any heatshrink and other stuff you want on the wire.
The big one is ground - the other two are tight and left channels. I prefer passing the wires through the inside, then soldering the outside . Clamp the jack pointing away from you, with the Ground connecter below. The small pin to your right is right, the one to the left is left. Solder these in place.