Set up your NeoPixel sticks in a pair of helping hands with the input ends facing you and close to one another.
Tin the pads with solder and connect one ground to another with a small piece of silicone coated wire. Also solder on a long wire to another ground pad, to extend to GEMMA.
Similarly solder up parallel connections between the pairs of digital inputs and power pads according to the circuit diagram. Take care where two wires are soldered to the same pad to avoid shorts.
If you’re making a headband, snip a hole in the outer layer of ribbon at the front center and thread the long wires through to the back of the headband. Use a seam ripper to open up the zigzag stitching a bit to get the wires out and make room for the GEMMA and battery. Solder the three wires to GEMMA according to the circuit diagram.
If you’re making a baseball cap, cut a hole in the front with a seam ripper and thread the long wires through before soldering to GEMMA.
Upload the rainbow sample code to GEMMA with the Arduino Code or to the GEMMA M0 using the CircuitPython Code. This will allow confirmation that your solder joints are solid before sticking the NeoPixel sticks together, back to back, using double-stick foam tape.