2 cups cornstarch
1 1/2 cups water
a few drops of food coloring
1. Mix all ingredients together in a bowl. These measurements are approximate. I've never actually measured them. Just keep adding and mixing until you get the right consistency. It should feel powdery and wet at the same time. You should be able to grab a handful, break it like a solid and then have it melt between your fingers.
Here's a video to give you an idea of what it should look like.