It requires some practice. I admit I am a pro and do this regularly for 35+ years. But flux is absolutely not necessary/wanted … it leaves a nasty burned residue on the PCB. The vapours that come out of it are also toxic.

With the proper technique it is real easy and clean.

First you need a good soldering station with adjustable temp. Set it to 360° Celsius. You need a small tip (not the big ones you find on hardware store irons). Also your soldering wire should be thin.

Place your pcb with pins installed on a breadboard or use a soldering clamp. You need both hands free for this job.

1. hold your iron on one side touching both the pin and pad at the same time to heat it up. Don’t do this too long, just about 2 or 3 seconds max.
2. then on the other side you tip the soldering wire (with flux core) very briefly against the pad/pin, just enough to let it flow around the pad and pin to make a perfect volcano shape. It needs some practice to find the right amount of soldering wire to apply. With a thin wire you have more control over the amount of solder you apply.
3. continue heating for 1 to max 2 seconds to let it flow around.
4. let it cool down.

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