Insert and solder the center eight LEDs into the D20-D27 positions. Follow the instructions in the image below for twist/inserting them. Make sure that they are all straight, then solder tack one pin and check that they are still straight before soldering the other pins. It helps to look at the LEDs on the board from both sideways and the top to make sure that they are all aligned, the same height and facing straight ahead.
Once you have soldered them, now is a good time to go and trim them (otherwise it will be very difficult to solder the next columns of LEDs).