Here's a downloadable and printable pattern. Print each of the 3 pages on letter sized paper, then tape them together to get the whole shebang aligned.
Adjust to fit your measurements until you've got a ringer. On me, floor-to-knees is 22" and halfway around my knee is 7.5". My spats come to just below the knee, but yours can be any size that makes you feel dolled up.
Figure out which way your fabric wants to stretch. Lay the pattern on the fabric so the stretch will end up going around your leg and the non-stretch direction spans the knee-to-floor length. Cut two of the pattern shape for each leg (so 4 copies total for 2 spats).
Stitch or serge along the straight cut line (the one next to the fold line in the pattern. I've allowed about 5/8" seam allowance, but since the fabric is stretchy this isn't too critical.
Take the seam you just made and lift it up off the table about 3/4 inch. Fold it over, making a doubled-up section that will act as the pocket for the LEDs. Pin this flap in place.
Stitch through all layers along the original seam line. Flip your spat over and stitch again along the outside edge of the fold you just made. This will secure the pocket on both sides, as well as giving you a nice tailored look.
Test-fit your LEDs in the pocket by slipping them in from one side. They should fit snugly without slipping around too much. Pull them back out again for now.
Cut a small square of fabric for each spat that's the right size to hold the battery and Gemma M0, so they won't rub against your leg. Stitch the pocket along the inside of the spat near or behind the LED pocket, leaving the top open.
Fold the spat in half with right sides together, and stitch or serge the opposite side. Try it on to be sure it fits. Once you're happy with the fit, make a narrow hem along the bottom. Make another hem along the top, leaving the battery pocket and LED pocket open so you can get the electronics in.
Be sure to use a zigzag or stretch stitch for the hems since this will pull when you put the spats on.
Slip the LED strip into their casing, being sure they're all facing outwards. Sew the Gemma inside the battery pocket using the unused pads. Wrap a rubber band around the battery and switch connector, trapping the battery leads inside. This will add some strain relief and keep your battery leads from breaking off, which can be a problem with these types of batteries. Slip the whole assemblage into the battery pocket.
Finally, glue some embellishments on the outside of the spats above each light. Buttons work great here, or metal gears from the scrapbook section at the craft store (if you're going for Steampunk). I settled on black leather Adafruit logo stars, cut on the laser cutter at my makerspace. Just use whatever irons your shoelaces.