the trick is to make sure they are clean, get them hot and leave the soldering iron on there for a while mix the solder and flux while melting it onto the brass tube. leave the iron on for a longer period of time to cook it and make sure it doesn't get gummed up inside the tube.
my soldering iron is decades old I got it from my pop and he used to work with electronics in the army and is long since retired.
I have never soldered anything and made a mess.
