you need a sharp tip soldering iron and a steady hand. Maybe also a sharp pickup to hold the LED in place.
I put some tin drop on to the pads first beforehand to make it easier to apply more tin drops and apply heat to the pad.
Once you get a corner secured, the rest are easier. Just heat the tin drops already on the pads and solder the LED feet on the pad. Check for cold solder after each one by plugging in the keeb and see that the LEDs light up. Oh ya, start from LED No.1 since they are somehow wired in series. If you skip, you can't check the LEDs until you've finished them all.