Add instant pudding and 1/2 cup of cornstarch in a medium bowl, then stir to incorporate.
Add the warm water and stir until all the ingredients come together and form slime.
Carefully add the remaining cornstarch to the slime.
Once it becomes too difficult to stir with the spoon, use your hands to knead the ingredients.
If the slime is too thick, add a small amount of water until desired consistency is met. If the slime is too sticky, add a small amount of cornstarch until desired consistency is met.
Notes
It is completely safe to eat, but should be a one-time use slime. Do not store and keep to play with again.