How to solder WS2812b LEDs(individual,not a strip) to a PCB?
I assume you don't have an SMT rework station :)

Put some solder paste on each pad. Place the part on the board. Use a toothpick or something similar to hold it in place, and use your iron to melt the soldering paste on each pad individually. I would guess about two cubic millimeters. If you are not happy with the result, you can touch up each pad individually. Use plenty of flux for the touch-up.

Practice on a scratch board with a dead part first, as it's easy to mess it up if you haven't done it before (but then again, how would I know :) )