The Sensor Watch circuit board is designed to fit in a Casio F-91W or A158 wristwatch. Note that it will not work with counterfeits! You need a genuine Casio F-91W wristwatch to serve as the donor watch for this board swap.
At a minimum, you will need a small Phillips screwdriver to perform this board swap. Tweezers can also help. Optionally, you may use a soldering iron to remove a small metal tab from the original F-91W circuit board, and solder it to the Sensor Watch board. This modification is necessary to get the buzzer to beep. Note that if you don’t do this swap, Sensor Watch will still work the same, with the exception that the buzzer will not sound.
You can view a video of the full board swap procedure here: