To decorate a Christmas fireplace, you can add some holiday cheer to the mantel with a mix of greenery, ornaments, and lights. Start by using a garland to drape over the mantel and hang ornaments along the length of it. Add some extra touches of greenery, like pinecones or holly, for a more natural look. Finish it off with a few candles for a warm and inviting atmosphere. For an extra touch, consider adding a string of lights along the top of the mantel for an added glow.