The pixels are chainable - so you only need 1 pin/wire to control as many LEDs as you like. They're easy to sew, and the chainable design means no crossed conductive threads. The output of one pixel connects directly to the input of the next.
To begin sewing, stitch around the data pin of your microcontroller (A1 on Circuit Playground, D1 on GEMMA or Gemma M0 and D6 on FLORA are ideal because they're right between power and ground), and stitch over to your first pixel.
Make sure that each arrow points away from the microcontroller in a line. Stitch around the input pad tightly, even knotting the thread here to form an extra sturdy connection.
Stitch back to the thread origin and tie the two ends in a square knot. Use clear nail polish to seal this knot and pull the ends tight until it dries. Do not clip the thread tails until later on.
All the positive pads (marked with a +) connect together to form one power bus. Likewise all the negative pads (marked with a -) connect together to form one ground bus.
Here's what a three-pixel circuit looks like from the back. See the individual data connections in the center and the continuous power and ground buses above and below?
Double check your knots are secure before clipping all your thread tails. Clean up your work space so there aren't any stray bits of conductive thread hanging around.
Visually inspect your circuit to check for shorts or stray threads.
Plug your microcontroller into your computer with a USB cable. Change the number of pixels in the Arduino sketch or MakeCode project to match the number of pixels in your projects, and make sure the pin number matches what you sewed to. Upload the strandtest sample code as you did when testing a pixel earlier in this guide. Your pixels should light up and animate different colors and patterns.
If they don't all come on or some later in the chain are flickering, your stitches might not be snug enough against the pads of the circuit board. Double check your sewing and reinforce it where necessary (with the circuit off/unplugged).