Bring the broth or stock to a simmer at medium heat on the stovetop. Once it begins to simmer, back the heat back down to low.
In a separate cup mix the cornstarch with the cool water. The cornstarch slurry will be stiff and hard to mix at first, keep working, it will loosen up.
Once the cornstarch slurry resembles milk, pour it by small amounts into the simmering broth and whisk while doing so. Heat for several minutes and the broth will thicken.
Add salt and pepper to taste.