Use a thread ripper to open the back seam of the slippers. Go a couple inches along the sole seam on both sides. Pull out the foam insole.

Slip the LEDs into the sole, making sure the lights are facing outwards. Push the foam insole back in to hold them in place. I found it helpful to trim the foam just a little bit to make room for the pixel strip. Try to get the strip in there evenly, so the middle LED is placed right at the front of the slipper. 

This is a little harder than it sounds. Turn the lights on once you have everything in place to make sure your LED strip is in there smoothly with no bumps or twists.

Slide the Feather into the right side of the slipper with the USB port facing the back (we need to be able to access it for battery charging), and the screw terminal facing outwards away from the feet. That terminal could make these slippers pretty uncomfortable if it's facing in and rubbing on tiny ankles.

Slip the FSR underneath the fabric lining and above the insole and secure it with a dab of hot glue when it's in striking range of little feet.

Slide the battery in on the left side, and slide in any extra wire from the switch. I found it easier to push the switch wire in first and slide the battery in afterwards. 

Keep the switch out and accessible. Sew up the back seam and the sole side seams with a needle & strong thread -- I used heavy duty thread since these will probably take some abuse.

Use hot glue to secure the switch to the back of the slippers where it can be easily reached and pushed by little fingers. 

Feel around for the USB port on the Feather. Use your thread ripper to cut a small hole in the fabric that will admit a USB cable for charging the batteries.