I have seen these, but never a "how-to" so I figured I would just play around a bit and see if I could duplicate the effect. It was quite easy! First, I used a craft knife to slit the papers on the crayons. I will save some of the wrappings for collage.
I used an old canvas that had a painting on it that I no longer cared for. I used black gesso to cover it up, leaving the textured circles that were part of that painting. Using gel medium, I glued my selected crayons to the top of the canvas. I did try to put pleasing color combinations together, but I think random would work just as well. Or even all greens and blues would look great on the black.
I put the canvas on an easel, and used my heat gun to melt the crayons. It doesn't take much, and it was fun to watch the drips. If I angled the heat, I could get splatters across the canvas too. Be sure to put paper towels or waxed paper at the bottom of the easel or on the floor to catch drips.
And here is the finished project. Some of the ends of the "drips" are not quite attached to the canvas, but the overall effect is pretty cool. Fun project.