When using an iron, I clamp my pcb in a vise-grip type C-clamp with leather glued to the clamp pads. That way the clamp isn’t stealing away your heat. I tin the pads and apply flux with a Kester flux pen to the tinning and the underside of the emitter.
Next, I crank up my iron to 840°F and hold it under the emitter using small circular motions. It takes no time at all. Use a fat tip for better heat.
The triple XP-G board I’m working on will already be attached to a copper slug, and because of the way I am trying to improve the thermal path on the aluminum PCB the LEDs will have to be left til last. I’m going to use PilotPPKs Method of floating the emitters into place.