What You Will Need:
-- some crayons
-- a microwave-safe container
-- some foil (or any shaped mold you want to use)
Step 1: Make Some Rainbow Goo
Toss them in the microwave and turn it on high for about 3 minutes. It may take more or less time depending on the amount that you melt, how big your container is, etc. In case you hadn't guessed, this mixture is going to be hot when you microwave it, so be smart. You may need to stir the mix to make sure that it is all melting, but be careful not to over-stir or you will blend all of the colors together and just end up with some ugly green/black crayons.
Step 2: Welcome to the Mold
If you decide to use the foil idea, just grab a pen or a crayon (if you happen to have any left) and use a few square inches of foil to wrap around it, making sure to fully cover the bottom and sides however high up you want the crayons to be. Next, grab a funnel (you can make one out of foil if need be), or think of another way to easily pour the crayon solution into your molds. Once they are full, stick your molds into the freezer to cool/dry them. You can just leave them out to dry, but it will take longer. A few minutes later (I would say 5 just to be safe) you can take out the molded crayons, peel away the foil being careful to make sure that the wax has fully hardened, and tah dah, you've got some rainbow crayons.
(Sorry about the quality of the pictures - they do look better in person)