Mix the cornstarch and water together with a spoon.
If the mixture is too dry and difficult to stir, add a bit more water.
Stir the mixture until it has a consistent colour and smooth texture. It should be easy to stir when you get moving, but will feel difficult and stiff when you stop moving.
Your oobleck is complete!
Video
Notes
If your oobleck is too watery and not forming a solid when you squeeze it in your hand, add more cornstarch, 1 teaspoon at a time.If your oobleck doesn't "melt" into a stream of liquid when you let it go, you need to add more water - again, add it 1 teaspoon at a time.