Find the end of your LED strand with the male connector. This should be the IN end of the strand. Double check this is correct by looking carefully at the back of the LEDs. Sharp eyes can spot an arrow pointing in the direction of data flow.
This is very hard to see. If you're not sure, start with the end that has a male connector - 90% of the time this is the IN end. If your lights don't come on down the road a bit, you may have a strip that's manufactured differently -- try the other end.
Cut the connector off with wire cutters and strip about 1/4" from each of the three wires.
Wrap the red wire around the Gemma's VOUT pin, the middle wire around D1 and the remaining wire around G. Solder the wires in place. Plug a battery into your Gemma and see if the lights come on. If they do, swell. If not, check the on/off switch on the face of the Gemma and make sure it's switched to ON. (It's easy to miss!)
If your lights still don't come on, try re-uploading your code or try using a different battery. If it's still not working, try soldering to the other end of your LED strand, you may have misread the direction of data flow.
Find the middle of your LED strand and cut the wires close to the 10th LED. Add the second Gemma M0 to the wires you just cut in the same manner. Test this strip as well, and if all is working, finish by trimming the female connector off the tail end of this strip.
You can use that itsy bitsy onboard switch as your main on/off switch if you'd like. But it's tiny and a little tricky to finger while you're walking around the juice joint, and truth be told, it will slowly drain your battery dry if you leave it plugged directly in -- even while it's switched off. Adding a larger physical on/off switch will keep your battery and your gin-addled fingers copacetic.
Find your tactile on/off switch and your battery cable. Clip the lead wires on the switch, and trim off both ends of the battery cable to about 2 inches.
Slip on some heat shrink. Solder the red wires from each JST connector to the leads from the switch. Solder the two remaining black wires back together again. You'll end up with a triangle shaped short JST connector line with a switch in the middle of the red wire, as shown.
Plug your battery into the switch adapter and the switch adapter into the Gemma M0. Click your switch and test to be sure the lights go on and off. (If not, check that onboard Gemma switch again, that cat can be a real hayburner).