Alright I'll take a shot at this...
1st: I'm guessing this mod is for a vtec light considering I don't think I've seen anyone else here use a relay to drive a single led.
2. LED's are polarity sensitive:
So with this information, take the cathode (negative side), solder a wire to it, and put a ring on the wire and bolt to a ground (usually a bolt that taps into the body somewhere). take the anode (positive side) solder a wire to it and connect the wire to the power source you want to use (guessing vtec wire).