Rinse and pick over the cranberries, discarding any bruised or damaged berries.

In a medium saucepan, combine the cranberries, sugar, diced apple, and cinnamon stick.

Place the pan over medium-high heat and bring to a boil.

Reduce the heat to low and simmer, uncovered, for 20 to 25 minutes, or until thickened. You'll hear the cranberries popping as they cook.

Remove the cranberry sauce from the heat and let it cool.

Transfer the cooled cranberry sauce to a covered container and refrigerate it until it's time to serve.