Applying solder

I have shaped the end of my middle sized soldering iron to make one side a flat surface about the size of the conductive patch. I bought some liquid rosin flux that I apply with the little paintbrush. Dab the iron onto the stick of solder and get it to the pad area as quick as you can so there is still flux from the core burning off of it, then dab it down so that it just touches the pad for an instant. The cell flexes and even crackles a bit sometimes as the heat causes expansion, so I wouldn't try and hold it down too firmly or it will snap.

Applying the iron for too long seems to actually burn off the conductive film and ruin the pad.