If you don't have solder paste or hot air: Tin the PCB pads, and the parts pads but only with a very small coating, then flux on both and heat up each pad in turn. If there's too much solder it won't sit flat.
If you have hot air: do the above but gentle heat the PCB until the solder on the pad gets a tad shiner - put the part in place and heat that - surface tension will move it into position once the solder on the part and the PCB have melted.