Add flux to second pad. (Photo 1)

Hold iron onto the second end of the diode and feed solder onto the board right at the tip of the soldering iron. (Photo 2)

Revisit the first end of the diode and repeat the step above to ensure a sufficient amount of flux is on the pad. (Photo 3)

Use isopropyl alcohol on a q-tip to clean any excess flux left on the board.