Measure the size of your cuffs. Mine are around 12 inches around and 8 inches high. 

Cut two pieces of fusible interfacing to this size for each cuff, so, 4 pieces in total.

Use glue dots or pins to lay out the lights in a zig-zag (serpentine) pattern on the sticky side of one piece of interfacing. I like using the 3d glue dots because they go on quickly and diffuse the light beautifully. 

Try to get the lights in an even square grid pattern, though if they're shining through thick fur this isn't too important - it'll look best with lots of lights so if you want to sacrifice the evenness of the pattern to squeeze in one more row of lights, go for it. My grid ended up with 7 lights across and 5 down. Once you're happy, trim off the rest of the light strand.

I laid out my left cuff with the lights facing upwards and the wire entering from the upper left. Then I laid out the right cuff as a mirror image - with the lights still facing upwards I have the wire emerging from the upper right. 

Both cuffs are connected to the same data pin so they'll play the same animation. By reversing the layout on one cuff, I get an automatic mirror effect between the two cuffs.

Place your second piece of interfacing sticky-side down on top of the light grid. Iron the pieces together. Be careful going over the glue dots - I focused on ironing between the rows of lights rather than right on top of them to minimize melting the glue. Be sure to iron all around the outside edge as well.

Plug your light strands in via the connectors and test to make sure all the lights are working before you install them in the coat.

Write down how many lights you have and in what configuration. You'll need this number (35 lights, set up in a 7 x 5 grid, in my case) to set up the 2d matrix.

Make a small hole in the cuff seam with your thread ripper. Slip the grid of lights up inside the cuff and wiggle them around until they're laid out nicely, with the edges of the matrix aligned with the sleeve seam.

Sew a few bar tacks through all layers at the corners and edges to keep the matrix from shifting.

Thread the wire extension up through the sleeve between the lining and coat, across the shoulders and into the hood. Plug the connectors in to the Mini Sparkle motion - use the two connectors soldered to pin 33.