affix the LED to the muzzle of the toy cap gun.

wire the LEDs back into the handle area.
bore out the handle, and insert the microcontroler, battery, LED pins, and a trigger.

cover up the bore hole where your electronics are stashed.

trigger the muzzle flash for when you pull the trigger. In future projects, I might use a micro phone to trigger the flash. Right now, I'm using a button, and its kind troublesome.