If you find that the chin rest inside Chewbacca's jaw is getting in the way (this may have been sized for kids...) you can unscrew it now.
Position the Audio FX board with it’s upper left mounting hole over the masks screw hole that originally held the plastic bracket you removed earlier. Use that original screw to screw the board securely to the mask. The USB port on the Audio FX board will still be accessable for file transfer and battery charging.
Tuck the battery underneath the board. It should sit nicely there, but you can use some double stick foam tape if you like to be sure it won't rattle around later.
Route the tactile switch so that it'll be accessible through the former battery compartment when the lid is open.
Next, you'll mount the selector switch. Use your diagonal cutters to remove the small plastic vent tab from the inner mask’s upper left hand vent hole.
Push the threaded shaft of the selector switch through this hole from the inside, and then screw it in place with the supplied nut.
If the wiring gets twisted up, sping the mask to untwist it. (Sort of like Spin the Bottle, except with more Wookie face and less kissing.)
You can then place the selector knob on the shaft.
Reattach the jaw, making sure to fit the return spring end into its place.
Press the mask back together, being mindful not to pinch any of the wires.
Screw in the eight Philips screws.
Now, push the tactile switch wiring back in and stick it in place with some double-stick foam tape (I'm using a temporary loop of gaffers tape in this example).
Note how you can access the USB port, and see the LED light up when the mask is powered on. You can go ahead now and snap the battery compartment cover back in place. Your mask is assembled and better than ever.