Note

If there is no flat side marked on the pcb, just use a multimeter to find the ground. Usualy ground is the square pad = cathode.

Now what if there is no flat side on the led, no flat side on the pcb silkscreen, build guide sucks and both pads on the pcb are round (no square pad). Well still use the same simple trick: use the multimeter to find the ground and insert the shorter lead in this ground hole.