Sew the gemma onto the hoodie in a location that is relatively close to where you are putting the leds.

I put it on the inside of the left half of the hoodie.

Make sure you read adafruit's guide on sewing with conductive thread, especially the part about using clear nail polish.

Sew (using non-conductive thread) a small pocket next to the gemma for the batter.  Make it fit your battery, not too big, not too small.

Sew the leds, one at a time, connecting power, gnd and pin D0 from the gemma

Test power and gnd to make sure they are not shorted

Upload the program to the gemma

Connect the battery and enjoy.

Possible improvements coming:

Adafruit just released a heartrate sensor, that could be fun to add into this project.

Accelerometer could also be nice to add in.

GPS?  Maybe so, not sure how that would work though.