I've seen several different versions of this, from leaving crayons on canvas in a hot car (we tried it) to using a blow dryer to running crayons through a small hot glue gun. We had the best luck with the latter. The crayons fit perfectly in the glue gun. Peel them first, of course (my kids did this) and then run them through. You'll need to press on the end of the crayon, rather than relying on pushing the button. This will NOT ruin your glue gun. It works just fine afterwards with glue, but you might have some color residue.
It's now hanging in my living room, awaiting a "cool colors" companion canvas. :)