Homemade Christmas cards: The evil mad scientist laboratory way...
"Make your own edge-lit holiday cards using LEDs, plastic, paper, electrical tape, batteries, a pen, scissors, and a hobby knife.
For our LED setup, we're using a classic "LED Throwie" arrangement-- an ultrabright LED is attached directly to the leads of a CR2032 lithium coin cell. When the bright LED is pointed at the edge of the plastic, it travels through cleanly but lights up the areas that we've scratched.
So we have the light going into the edge of the plastic. Next we need to (1) hold the LED there and (2) restrict light from going in directions that we don't want. We can accomplish both of these goals by using black electrical tape to hold the LED in place. We start by putting down a piece of tape, sticky side up, and sticking the plastic plate and the LED to it. Next we add a matching piece of tape on top, forming a tight seal around the LED but keeping it pointing into the plastic.
If you insert a piece of black paper behind the clear plastic, you might find that it improves the contrast".
Source: Evil Mad Scientist Laboratories