Step 10: Prepare the sensor plate
To connect the sensor plate to the PCB, you'll need a short length of wire. I would recommend at least 3-4 inches. Extra doesn't hurt, since everything will be concealed inside the box.
Strip about 1/4" off of each end of the piece of wire, then pick a spot on the copper side of the sensor plate where you'll solder it. You can solder the connection anywhere along the edge of the plate. I put mine in the middle of one side, but a corner would work just fine, too. The sensor plate is a big piece of copper, which makes it hard to heat up a particular spot. If you're using an adjustable iron, it's nice to jack the temperature up a little bit here.
First, just heat up the spot on the board and apply a little solder until you have a solder blob on the board. Then, while keeping the solder blob liquid, stick one of the stripped ends into it, then remove your iron. Once its cooled, you should have a nice joint. Try to use the minimum amount of solder that gives you a good connection - the more low profile this joint, the better.