By the end of the year our crayon box is looking pretty tired - lots of stubby ends and broken pieces - quite a lot like this, actually!
There is a tonne of information out there on how to do it. Here's an example. I don't think you're supposed to heat them, but I used ice cube moulds for mine and melted the crayons on a very low heat and they turned out just fine.
Packaged up for friends - they make a lovely scrappy gift!