I sewed the snap contacts on a shirt with the conductive thread, and used the thread to connect the neopixel 12 LEDs ring. I also soldered the second part of the snap buttons to the flora pads, so I now can snap it on and off. And it works pretty well :)
Side note: I kept an other shirt underneath because I didn't want my body to make a short and damage the Neopixels orthe FLORA board.
I received the MAX30102 pulse sensor / oximeter yesterday, so I ran into my room to try it out with the FLORA board.
I plugged it in, using the I²C pins (SDA and SCL), and powering it with the VBATT and a GND pin. I used the sample code in SparkFun's library, modified it a little so Flora's onboard LED would beat to the rythm of my heart. The result is not really good, I will try to solder the pins on the MAX30102, isolate them, and put the sensor on other veins, to find where the beat is better detected.