Create the circuit and program the arduino
The circuit is simple: POWER + ARDUINO + (2) SERVO (in swing mode). The Arduino program is a bit more complicated. I wanted two arms moving randomly and independently of one another, to give it more creature-like verisimilitude. The basic algorithm for a motor was:
delay (random 5-10 seconds);
run (random 50%-100% speed) for (random 2-5 seconds);
Because the two motors were running independently, I couldn't use the delay() function. So I had to keep track of the delay/run intervals myself in variables, and each time through the loop calculate where we were in each cycle using the millis() function. The Arduino sketch is uploaded as an "Additional File" in the right sidebar.
Build the servo motor mount and arms
The servos are mounted in a cardboard triangle. Cut opposing slots at the top of the triangle to fasten the ends together, and cut holes to mount the servo motors in on either side. Make sure the base of the triangle is wide enough to accommodate the mounting board with the circuit, and place the board inside the triangle and secure it with tape.
Build the headpiece and cell phone holder
Make a circle out of cardboard that will fit snugly on your head. Make sure the corrugations are vertical, as you will use the channels in the cardboard to hold the wire. Run two wires front-to-back to make a base for the framework, inserting the ends of the wires into the spaces in the cardboard. Put another piece of cardboard over the wires to make a base to mount the circuit assembly. Cut a piece of cardboard and wrap the ends to hold the phone, and leave a small flap to fold up to hold the bottom of the phone. Secure with some tape. Again, the cardboard corrugations should be vertical. Then run wires through the cardboard and into the headpiece to hold the phone at the front.
Mount circuit assembly and phone holder
Tape the circuit and motor assembly to the headpiece, making sure there is clearance for the phone holder. Tape the battery to the mounting board.
Cut eye hole in hat
Put the cap over the whole contraption and cut a slit in the hat where the phone (eyes) are. The hat needs to be very lightweight and stretchy because the servos are not that strong. I tried a regular knit hat at first, but the motors seized up. We ended up making a hat out of lightweight jersey fabric (thanks Tina!). Cut a slit where the eyes will be; the fabric will stretch so it doesn't need to be a large hole. trim a little extra fabric if needed.
Turn it on and bring the creature to life!
I used a phone app from Morph Costumes called Digital Dudz for the eyes. They have an app that does a number of creepy eyes; I used the not-quite-so-creepy Cute Cat. I also grabbed some sound effects and played them in a loop on the phone--bear growls and pig grunts tweaked up nicely. I suppose you could use the littleBits mp3 player and speaker, but there wasn't that much room left under the hat! Turn on the eyes, turn on the sound effects, slide the phone into the holder, turn on the littleBits circuit, cover with the hat, and place the creepy creature on top of your head!