To open the spare battery case, slide the dark grey cover in the direction of the molded arrow, and set it aside.

Also remove the white button cover. The batteries have a tendency to all out during handling, so keep a keen hand ready or take them out and set them aside as well.

Lift the small white lever to expose the printed circuit board inside.

Take a look at the cut end of your excess ribbon. See how every inch or so, there's an exposed section of conductive thread? That's the part you want to insert into the battery box, so trim the ribbon to position an exposed section near the end.

Flip the ribbon so the exposed threads are facing down, and slide the ribbon into the battery box as shown. Close the lever over the ribbon, insert the batteries, and press the button to test it.

LEDs are polarized, so the ribbon will only work when plugged in one way. If it doesn't light up at first, try flipping the ribbon over. Plugging it in backwards won't damage the LEDs.

If your ribbon lights up, replace the white button cover and outer grey shell. You're done! Now you can use the ribbon for another project, or the other shoe!