Get good quality electrical solder (not fux cored) Relatively expensive but will probably last you a life time.
Contrary to many (as always) I always use an electrical soldering iron. Plumbers flux. Tin the soldering iron.
Once you have the nipple in the correct place, unravel the 5mm excess you should allow for, bend ends over and use a blunt end punch to spread the strands. (cat's a*se) Put small amount of flux onto Cat's A, heat with (pre-heated) soldering iron, introduce solder and allow to flow through joint, a little. Allow to cool, clean up with grinder or file.