The Monster Box set-up consists of the scary box itself and a control box.
Inside the Monster Box is a servo motor that has an arm which swings up to open the lid.
The control box holds the Circuit Playground Express and a battery pack. This will help protect the electronics and make it a little easier to arrange the elements of your light-sensor-triggered box where you want them.
To prepare your box, make sure the lid lies as flat as possible. If it keeps popping open a little, try bending it back and forth along the fold line.
You can use also try scoring along the fold line by making an indentation along it with a rounded point (such as a dried-out ballpoint pen or the handle of a spoon).
Next, to make the teeth, sketch out a zigzag line along the inside of the lid's edge. You can make the teeth jagged, but keep the tops of the teeth even along one "gum line" for the best effect.
If you are using the micro servo, it helps to cover the labels on the sides by wrapping some masking tape around the body of the servo. Otherwise the labels have a tendency to peel off when you try to glue the servo down. This also makes it easier to remove the servo if you wish when you're done with the box.
Attach the Servo Horn
Find the one-armed servo horn and press it firmly onto the top of the servo.
Then remove the horn and reposition it so it lines up with the body of the servo. It should now swing in a complete 180 degree arc from front to back.
(If you are using the standard servo or another servo that doesn't have a one-armed horn, use the smaller round servo horn, or any shape that lets you lay the servo flat on one side.)
Make an Extension For the Servo Horn
Use a piece of scrap cardboard to make an extension that can push the lid open. To make it tall enough, you can glue the cardboard to a wooden craft stick, like a flag on a pole.
Then glue the extension onto the servo horn so that it will point up when it is attached to the box.
Lay the servo in the box so the end with the wire is pointing towards the back. The extension you connected to the servo horn should lie flat along or just above the bottom of the box, pointing towards the front.
Make sure the extension isn't keeping the lid from closing. If it's too tall, trim as needed.
Use adhesive dots or other tape or glue to attach the servo securely to the bottom of the box.
Add a Hole to Thread the Wires Through
Mark a spot in the back of the box near the servo wire. Carefully cut out a hold big enough to thread the wire through.
Take the smaller control box and hold the battery holder against one side so that the on-off switch presses into it. Cut a hole around that spot big enough to let you slide the on-off switch back and forth. Attach the battery case to the side with adhesive dots, lining up the switch with the hole.
Attach the CPX to the Top of the Control Box
Press two matching Velcro dot halves together. Attach one side to the back of the CPX. Position the CPX on the top of the box and press down so the other Velcro half sticks to the box.
Connect the CPX to the Battery Case
Cut a hole through the top of the box near the JST plug.
Thread the wire from the battery case through the hole. Plug the wire into the CPX.