To make homemade butter for breads, baked goods and other uses, simply fill a jar halfway with heavy cream, screw the lid on tight, and shake the jar up and down until the cream thickens and starts to stick to itself.
According to About.com's Frugal Living section, baby food jars work great for small batches of butter. The shaking should take about 10 minutes depending on the amount of heavy cream used and how hard you can shake the jar. Once the cream has thickened, open the jar and drain any remaining liquid into a new container. The liquid is buttermilk. Knead the butter (in the original jar) under cold running water for a few minutes to work out any additional buttermilk. This prevents the butter from spoiling too quickly.
It's optional to knead in salt or fresh herbs like garlic, parsley, chives and rosemary, which also make tasty additions. You can drink the leftover buttermilk or use it in pancakes, baked goods, in a creamy salad dressing or to coat chicken for frying. Homemade butter should be refrigerated when not in use. Heavy cream can be found in the dairy section at most supermarkets. Fresh cream will produce sweet butter.