1. Put a little solder on one pad.
2. Put the LED on top and apply flux
3. Use the iron to reflow the solder and gently push down the LED (using tweezers) so it sits flat on the board and now makes a connection
4. Solder the other side.
1. Put the LED on the board and hold in place with tweezers.
2. Apply flux.
3. Put a little solder on the iron then touch it to the pad so the solder flows onto the LED and the pad.
4. Solder the other side
5. Go back and re-solder the first side if necessary (or just add a little flux and reheat with the iron)
: compatible with Arduino UNO, 24V operation, all I/O's fused and protected