Follow the directions on the box of red Jell-O
I used the "quicker method" the box refers to, using ice to speed up the process
Pour a small amount of the liquid into each cup
Refrigerate until it has set completely
Continue with the next color of the rainbow
Add a small amount on top of your first color
Refrigerate until it has set and then repeat each step again until you have used every color of the rainbow
Once all your layers are looking pretty, add some whipped cream and sprinkles!