i dip my wire in flux then lay it on the pot take my iron hold it just above the pot add solder till it just about drips off then roll it on to the pot maye ted will do vid hes has a video on here about soldering piezos
Tape the wire on temporarily to keep things from moving around. Also have the pot affixed to something like one of those third hand tools, or drill a hole in a board and stick the shaft into the hole.
The lead free solder requires a hotter iron, and so does heating the larger mass of the pot, but 30 watts is supposed to be sufficient - 50 watts - wow. You might be in danger of overheating the pot if you leave the iron on too long.
Hold the iron on both the wire and the pot for a second to heat them up, then feed a bunch of solder into it, until it melts into a puddle surrounding the wire. That would be about another second. Then remove the iron quick, so you don't blow the pot.
When the solder cools and solidifies, you can remove the tape.
I don't use extra flux.
Thank you Skeesix i like the idea of taping the cable down and a third hand vice. I have some soldering to do tonight i will let you know how it goes.