Here are the things you'll need to make this:
1/2 tsp borax
2 Tbsp warm water
1 Tbsp glue
1 Tbsp cornstarch
In one bowl mix the borax and water- add food coloring to this if desired. Mix until dissolved. In another bowl pour glue cornstarch, and 1/2 tsp of the borax/water solution. Do not mix, just let it sit for about 20 seconds to interact. Then stir together until fully mixed. It will be a sticky mess.
Store it in a plastic bag when not being played with. DO NOT EAT- borax can be toxic.