We recently made heart shaped crayons using a silicone baking sheet of hearts and old broken crayons.

Strip paper off of crayons
Break into small pieces
Place into silicone molds (I easily cut mine in half to fit into toaster)
Place on sturdy tray. Heat in oven at 250
Check constantly
When pieces are melted take out of oven immediately
Let cool completely
Pop out of mold by turning the sheet inside out

We used the toaster oven and heated the crayons out on the porch just in case the smell of the crayons melting became overpowering.

One quick tip: You may notice some of the crayons are a solid color and some have many different swirls of colors. We prefer the swirled colors. The solid colors heated too long and the colors melted together too completely. To get the swirled effect take the crayons out of the toaster as soon as they are melted.