Tin the pads

Apply a small amount of flux to the battery terminal pads. (Photo 1)

To a clean iron tip, apply a small amount of solder. (Photo 2)

Place the tip of the tinned iron onto the fluxed pads until a small blob of solder is left on each pad. (Photo 3)

Use isopropyl alcohol on a q-tip to clean any additional flux left on the board.