Cream is simply milk with a lot of fat in it. "Heavy" cream is a particularly rich grade of cream. In contrast to skimmed milk, which has almost no milkfat, heavy cream in the United States is required to have at least 36 percent milkfat—making it the richest grade of milk widely available at the retail level. This high fat content makes it less suitable for drinking, but is excellent for use in many dishes.
Find a vendor that sells non-homogenized milk. Milk essentially is an emulsion of tiny fat globules and water. Because fat is less dense than water, its natural tendency is to rise to the top. That's what happens in non-homogenized milk: The fatty cream rises to the top while the lean milk stays on the bottom. Homogenization prevents this separation process by making the fat globules so tiny that they cannot effectively de-emulsify. Most commercially available milk is homogenized, and you may have to go to a specialty store to find an non-homogenized option.
Buy about nine times as much milk as you will need heavy cream. That works out to a little over a half gallon of non-homogenized milk to yield 1 cup of cream. This is because non-homogenized milk has about 4 percent milkfat, and you will need that much more of it in order to get the enriched, 36 percent milkfat cream separated out.
Pour the non-homogenized milk into a bowl or other container with a top that is large enough to make skimming easy. If you keep the milk in a bottle with a narrow opening, you'll have trouble skimming the cream.
Let the milk rest in the refrigerator, covered, several hours or overnight. During this time, the cream will settle on the top.
Skim the cream into a separate container. At this point, it is ready to use.