Measure the diameter of the lens on the flashlight.
Find an image of the bat symbol online and save it to a graphics program.
Size the bat symbol image so that it will fit across the lense of the flashlight.
Print out the bat symbol and use it as a stencil to trace the image onto cardboard. Cut out the stencil.
Tape the bat symbol across the lense of the flashlight. Turn off the lights and then shine the flashlight with the signal.