Dump in (2) 4 oz. bottles of Elmer's glue --I used the regular washable Elmer's school glue. Fill the empty glue bottles with water; swish it around and dump it in with the glue. Add a few drops of food coloring for a little excitement. The girls loved stirring in the food coloring.
In a measuring cup, add a teaspoon of borax to a 1/2 cup of warm water. Stir well. Add the borax water to the glue. Borax can be found near the laundry and detergent products in most grocery stores.
At first, you will think you have done something wrong because the mixture will be stringy and watery. Keep working it. I ended up using my hands as it seemed to be the easiest way to mix it evenly.
Eventually, the mixture will coagulate. It can take 5 minutes or so.
Squish it. Squash it.
Our full product line. We used empty peanut butter jars to store it.
If you're interested in some of the science and history of the stuff, check out Steve Spangler's Science Experiments. He has a lot of other fun, simple science experiments that you can try out with your kids!
If you're concerned about the use of Borax in a child's play mix, you can read this well-balanced article on the properties of borax. Personally, I don't have a problem with it.