Add instant pudding mix and half of your cornstarch to a mixing bowl. Add 1/3 cup water and stir until slime begins to form (it will be pretty sticky at this point)
Slowly add the rest of your cornstarch, stirring as you go. When slime thickens and is hard to stir with a spoon, finish kneading by hand.
If slime sticks to hands, add more cornstarch until you have a soft, malleable dough that stretches.