The audio/video TRRS (tip, ring, ring, sleeve) socket has the analog video signal on it. Don't try converting HDMI to composite, that's a waste of time (and the cheap cables won't ever work).
Since your RPi already has composite you just need the right sort of cable.
T == L audio
R1 == R audio
R2 == GND
S == video
http://www.raspberrypi-spy.co.uk/2014/0 ... ideo-jack/