Stir together gelatin and cornstarch in a bowl. Then stir in the warm water slowly.
Once the Jello starts to turn into liquid, stop adding liquid. Continue stirring and test consistency. For a thinner slime, add a bit more water 1 tbsp at a time
If you want it more like playdough add less water so it is stiffer in consistency. Knead until it is right for you and play away.