Tilt and rotate the back so that the Circuit Playground Express can fit into the opening.
Press the back piece so that it snaps into place.
If you prefer a way to turn the circuit on and off without popping off the back of the heart, you can easily solder together a small switch for this purpose.
Create a small power switch for the battery.
Solder two wires to the slide switch. Insert these wires into the heart through the aorta.
Cut the ends off a JST extension cable. Strip the tips and solder the black (GND) wires together.
Solder the switch so that it connects the red wires together. You now have a small and effective power switch!
With an external switch you can now turn on/off power without having to unplug the battery.