Press the iron's tip against the circuit board's LED solder pad for a few seconds to heat it, then feed the solder wire into the heated area until there's a small glob of solder on the pad.

Place the LED in the outline labeled LED on the board, with the white (non-green) side aligned with the "+" symbol.

The green markings indicate the negative (-) side.

Solder the LED's first end cap to the board.

The LED's contact pads are underneath it, so the solder joint doesn't need to cover the entire end.