The battery will come with more wire than what is needed so you will need to cut the wire down to about 4 mm long

Tin the Wire (Link to Helpful Guide for Stripping and Tinning Wires)

Use a pair of side cutters to strip away about 1 mm of insulation.

Apply a small amount of flux to the now exposed wire.

To a clean iron tip, apply a small amount of solder.

Place the tip of the tinned iron onto the fluxed wire until the solder wicks into the wire.