Step 1: The Pattern
Fold and rub along the line so that the carbon from the pencil lead transfers to the other side. This gives you a symmetrical heart.
Darken the faint outline, too, so that it is easily visible. Make any corrections as needed.
Now you have the paper pattern for the heart.
Step 2: The Plastic
Make rough cuts to approximately match the lines of your pattern, then remove material closer (or on the line) on a second pass. If you have a jigsaw you might be able to do it in one pass, but I was using a standard hack saw.
Then use a rough file or rasp to remove more material and smoothen the edges. Then use this as a pattern to make a copy on the second sheet of plastic: I used a sheet of opaque red perspex 3mm thick.
So there should be two sheets of plastic, each cut in the shape of a heart.
Step 3: The Circuit:
With no resistor, the LED is brightest but the life is short. I selected a 220 ohm resistor as a compromise between reasonable life of the small battery and visible brightness.
Holes are drilled on the transparent sheet of perspex to accommodate the two cells and LED. The resistor is accommodated in a channel cut between the holes.
The components are soldered and pressed into the plastic so that the two sheets may be aligned and stuck to each other.
Remove the paper from the two surfaces of plastic, clean, and join them using superglue or chloroform. Clamp together for about an hour or so.
Then extract it from the clamps, remove the rest of the protective paper, insert the two cells as shown and hold them in place with some conducting springy stuff and a screw. The LED will light, and the edges of the transparent sheet will shine as well.
Present it to your chosen Valentine.