Cornstarch is made by an industrial process that home cooks cannot replicate in their kitchens. Cornstarch is a pure starch extracted from corn used by many home cooks to thicken sauces and gravies. Manufacturers rely on a complicated wet milling process to separate the starch in the endosperm from the outer hull and the germ.
The industrial process to make cornstarch begins by soaking corn kernels. Next, large milling machines loosen the hull. The resulting mixture is finely ground, then strained with a screen to separate out the starchy liquid. This starch "milk" undergoes a washing process, then is separated by centrifugal force and dried to produce the fine white powder sold at the grocery store.
Though a home cook can't make cornstarch at home, it's possible to make a good substitute: potato starch. Use an equal amount of potato starch in recipes that call for cornstarch, including soups, sauces, gravies and custards. To make potato starch, grate potatoes into a bowl and add some warm water. Strain out the starchy liquid by wrapping the gratings in cheesecloth and squeezing the liquid into a bowl. Refrigerate the liquid for four hours, then pour off the top layer of water that has separated from the starchy liquid at the bottom. The starchy potato liquid can be used directly or dried to a powder.