Add the heavy cream to your food processor and blend for 10 minutes.
You will see the cream begin to change properties and separate from the buttermilk. It first starts to look like whipped cream. Then it begins to separate and look like more of a curdled milk. Then it becomes more solid.
After 10 minutes, pour off the buttermilk.
Then, press the butter into a fine mesh strainer or cheesecloth (or you can even use your hands) to remove as much of the remaining liquid as possible.
Next, return the butter to the food processor and add 2 tablespoon of cold water to remove any remaining buttermilk. The butter will last longer if you get as much out as possible.