Download the code and open it in Arduino. Connect the Feather to your computer and upload the code.
Disconnect the Feather from the computer. Connect the battery.
Clean your forehead with some rubbing alcohol to make sure it is free from dirt, oil, makeup, and lotion.
Attach 3 electrodes to the connectors on the sensor. Remove the paper backing from the two on the circuit board. Position the sensor on the forehead according to the image below. The wires should point up towards the hairline and the sensor should sit at a diagonal, with the lower end above the inside edge of the eyebrow and the higher end shifted away from the center.
Remove the paper backing from the third electrode and position it on your temple.
Open the Adafruit Bluefruit LE Connect App on your phone or tablet. Hit "Scan For Peripherals" and wait for the Adafruit Bluetooth LE to appear in the list. When it does, hit the "Connect" button next to it.
On the next screen select "UART".
Now you will see your sensor values on screen! Raise and lower your eyebrows to see the sensor values change.
If sensor values don't change try repositioning the sensor so that it sits more precisely on top of the muscle.