There are lots of different versions of this, and various techniques, including using a hair dryer, glue gun, or even an oven to melt your crayons.
My little one had to do a 100th Day of School project, which involved making a display of 100 things. We brainstormed some ideas, and she looked at a bunch of pictures before settling on using 100 crayons. She had hoped to make a heart shape, but we couldn't fit all the crayons onto the canvas we were using, so we decided to make 2 rows of crayons instead. This was a good exercise in the concept of "half," as she had to sort out the colors and use half of a given color on the top and half on the bottom. I put lines of hot glue onto the canvas, working small sections at a time, and she arranged the crayons into place. (ETA: This was meant as a family project. If you want your child to do it on his own, I'd recommend using tacky craft glue over regular white glue). We then put the whole thing into a large cardboard box to protect from wayward drips and splatters, but actually, it was all very tidy and we had no mess at all.
The crayons were melted using a hairdryer with the heat set on high and the fan speed set on low. My daughters and I took turns holding the hairdryer very close to the crayons (and obviously they were supervised during their turns). I was going to make a note of how long it took, but honestly, it was so much fun watching the process that I lost track of time! There are already a number of instructions out there in case you want to make your own. We used the one at 52 Kitchen Adventures as a reference.