Measure out how many neopixels you'll want on each side against your pattern. Carefully cut through the solder pads between pixels and slip the silicone sleeve off.
Tin the pads on both sides of the pixels at the "in" end. Cut two 6-8" red, white, and black wires. Solder the red and black to the front of the neopixel strip to + and -, and solder the white wire to the back of the neopixel strip (double checking again that you're on the "in" end).
Temporarily twist the matching wires from each strip together and twist them into the Circuit Playground's VBAT, 12 and G pins. Turn on the Circuit Playground and be sure all the lights come on and work. This is a great time to find any bad or broken pixels!