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.