Stick some insulating tape over the ECG contacts at the bottom of your wristband, since we are replacing this contact area with one of the electrodes.
Cut off the USB-A connector as we don't have use for it
Slide the big shrinking tube over the USB cable
Separate the wires as shown in the picture
Cut off all wires ecxept for orange and purple
Move the two small shrinking tubes over the ECG wires
Solder the ECG wires to the USB wires as shown in the pictures
Test your ECG setup on your Card10 prior the next step
Slide the small shrinking tubes over the soldering connections and apply hot air (e.g. lighter)
Slide the bigger shrinking tube over the two small ones and also shrink it
Have fun and hack ECG stuff
Place the electrodes on your chest as shown in the picture and connect the USB-C connector with the USB symbol downwards. After starting the ECG application on your Card10 press the right lower button (RIGHT) to switch over to ECG over USB.
If you want to start logging your ECG data press the lower left button (LEFT) to start and stop logging. Log files are written to the filesystem (ecg-<time>.log) and can be transferred to a computer using the boot loader. You can visualize them with Audacity: Use File→Import→Raw Data, select the log file, then use Signed 16-bit PCM, Little Endian, 1 Channel, offset 0, amount 100% and sample rate 128 Hz. Normalize the amplitude by selecting the entire track (Ctrl-A), then use Effect→Amplify and keep the suggested Amplification value.
To detect eye movements like fast movements to the left or right or to detect REM phase while at sleep, place your electrodes on your temples.