Solder a resistor on either leg (doesn't matter) - LEDs are polarity sensitive - that is, they won't light if you apply the voltage the wrong way around - no damage will result is connected incorectly, just no light! Then connect the remaining LED leg and the OTHER end of the resistor to your power source putting the resistor in series with the LED (the resistor limits the current that can flow through the LED and stops it from drawing too much current and melting).
I would suggest 220 ohm 1/4 watt for 6.3V and 1.2k ohm for 12V.
