MODDING 101: LEDs MADE SIMPLE
A typical LED will have a long pole and a short pole. The long pole is the anode or positive side, the short pole is the cathode or negative side. Some LED manufacturers have poles of similar size or you may be using a used LED, in this case there might be a flat side to the acrylic on the negative side. If there is no flat side and both poles are of same length then look for the “cup” that is also the negative pole.