I've seen this idea all over the internet, and last weekend finally got around to testing it out myself! It's a simple process--hot glue crayons to a canvas, and then take a hair dryer and melt the crayons down until you get the look you're going for.
There is a lot of splattering involved, so make sure you cover your space adequately! The good news though, is if you happen to get crayon wax on your mom's laundry room floor tile, it's pretty easy to scrape off once it's dry again. Not that I'm speaking from experience or anything.
I might be addicted now, and have already bought another canvas to experiment with. It would be cool to make canvas with only one or two color groups!