Pick out where the Gemma will sit on the jacket. I picked the location of where a breast pocket might be.
Using the thicker (26 awg) wires, cut 3 pieces to connect the pixels on the left to the Gemma and another 3 pieces to connect the pixels on the right to the Gemma.
Solder the larger and longer wires to first pixels, labeling wires if they are all the same color.
Using alligator clips, test the strand using the NeoPixel Strandtest code or other test code.
Note that if some strips have been flipped, a rainbow might appear a bit off. As long as the strip works, set it aside and test the second strip!
If you run into issues, turn things off, trace your data line, check your microcontroller, soldering & replace strips, as necessary.