If you bought a pre-made backup battery, you can skip this step and "follow these steps in reverse order".

Otherwise, if you have a CR2016, CR2025 or similar size/thickness coin cell, then read for how easy it is to tape the contacts to your battery.

cut open the sleeve and carefully rip away the tabs from the old battery.

Hold the contacts to the battery as you tape them together.

Red = positive | Black = negative

Don't put too much tape, but wrap it tightly. Of course don't do it too tight.